Which International Auxilliary Language (IAL) should you learn?

What languages are there?

• Esperanto (1887)
• in Esperanto, the name means "Hoper", i.e. "One who hopes".  The pronounciation in Esperanto is approximately "Es-pair-ONT-o".
• Originally developed by one man, Mr. L.L. Zamenhof, over a period of at least 10 years.  As originally published, it was a very simple language with only 16 stated rules and  Once published, it was further refined by groups of speakers who introduced new words and solidified the grammar rules.
• Of all IALs, Esperanto is the most widely known and used, having up to 3 million speakers (though I expect most of them are not fluent).  Obviously there's a lot of room for IALs to grow in popularity; to me, this means there is hope that a better IAL will take the throne someday.
• Fairly easy to learn, but laden with difficulties such as
• Plural adjectives (One red potato, Two reds potatoes)
• The mandatory accusative, in which the object (aka complement) of a sentence ends with -n.  This lets you put the subject, object, and verb of a sentence in any order.  It's great in theory, allowing one to use the same word-order as one's mother toungue, whatever that might be.  Unfortunately, in practice, it's really hard to learn to use it fluently.  Also, it's often hard to add the -n to proper names such as Paris, David, etc.  (If you want to do that, you normally have to "Esperantize" the word, and so Esperanto has its own names for things: Parizo, Davido, Nov-Jorko (New York), etc.  You don't have to do that, but without the -n, a sentence can be more difficult to understand.)
• Unpredictable word derivation across parts of speech: the meaning of a verb generally must be learned separately from the meaning of its noun because the relationship between them is unpredictable (although, on the plus side for English speakers, the relationship is often the same as in English.  But don't count on it.)
• Unpredictable transitivity
• A great many "single-word idioms": words composed of multiple roots that mean something different than you might expect.  Luckily, there are very few multiple-word idioms.
• Less "precise" than Ido; i.e. it is often difficult to precisely convey what you want to say.
• Too long: Esperanto usually requires more syllables than English, and sometimes much more.  In particular, some of the most frequently-used words that are one syllable in English are two, three or even four syllables in Esperanto.  While all IALs tend to be longer than English, Esperanto would probably win first prize.
  

• Latino sine Flexione (1903)

• A highly simplified version of Latin.  It looks like the Latin-speakers didn't like it because it "bastardized" their language, while everyone else didn't like it because they weren't interested in Latin.
• Apparently got a name change to "Interlingua" at some point, but we don't call it that because there is a language from 1951 with the same name.
• Ido (1907)
• The name means "offspring" in both Ido and Esperanto.  Pronounced "EE-doh".
• Ido is a completely overhauled revision of Esperanto.  It is different enough to be called a different language, but they do share a lot of word roots and affixes.  Ido's developers did not try very much to keep the same word roots as Esperanto; instead they did a somewhat extensive study of all the European languages to find the "best" root in each case.  Nevertheless, many roots are identical in the two languages.
• Ido has a richer set of affixes (prefixes and suffixes) which allows one to better express oneself.
• It seems to me that Ido has a more European bias than Esperanto, because it generally prefers to import a brand new root for a concept rather than to make a word from existing roots.  For example, the Ido translation of "forest" is "foresto"; in Esperanto we say instead "arbaro", meaning "group of trees".  In this way, Ido is easier than Esperanto for those that know a "European" language (especially a Romance language like French), while Esperanto is easier for everybody else.
• The word choices for question words (how, what, etc.) and pronouns (he, she, etc.) is more chaotic than in Esperanto and totally baffling to me as an English-speaker.  Apparently these words were chosen to match some language other than English.  Esperanto, on the other hand, has "regular" pronouns and question words (pronouns consistently end in 'i' and all question words start with "ki" except one.)  Oh, and here's something annoying: Ido has two-syllable pronouns for "he" and "she".
• The increased number of roots, and the less vagueness in word definitions, gives Ido more precision than Esperanto.  Thus Ido is better for technical writing (except that, owing to its few speakers, I suspect modern computer and other lexicon might be missing from Ido)
  

• Occidental (1922)
• The name means "western".  The name was later changed to "Interlingue" but this causes confusion with the two languages called "Interlingua".
• Occidental was the first of the "naturalistic" planned languages, which attempted to match the spelling of internationally known words as much as possible, while still avoiding the irregularities of the natural languages.  Perhaps, if you knew enough European languages, you might comprehend Occidental texts without studying it at all.
• Novial (1928, 1930)
• The name is an acronym for NOVi Internationali Auxiliari Lingue (New International Auxiliary Language)
• Novial is carefully designed to look "naturalistic", like Occidental, while keeping all the advantages of Ido and Esperanto such as regularity, flexibility, and ease of learning.
• The phonetics of Novial are easier than Ido and Esperanto; it banishes the letters "z" and "c" (except in "ch"); the difference between "z" and "s" is difficult for some to grasp, such as the Dutch, so in Novial, you can pronounce "s" as "s" or "z" according to taste.  In Ido and Esperanto, the "c" is pronounced "ts" as "tsar" and "hats", and is somewhat difficult for English speakers, among others.  Instead, "c" is replaced with "s" or "k" on a case-by-case basis.
• The pronouns are the same as the nouns in how they specify gender and number.  Novial is the only language I know of that unites nouns and pronouns this way.
• It's an interesting language and I'm a fan.
  

• Interlingua (1951)
• This is the only language to be created by a group headed by someone, Alexander Gode, who didn't actually like the idea of a universal second language for everyone.  Instead, it was intended for the scientific community.  Apparently.  I think he and his team were paid to make it.
• Interlingua is the height of naturalistic design, and prizes above all else resemblance to European languages.  It is easy to learn to read for those who know a European language, especially a "Romance" toungue.  However, it is the most difficult constructed language to learn to write because it is highly irregular and has a bunch of unneccessary, complicated rules.
• Interlingua is a written language only.  It has no phonetic rules.  Rather, you're just supposed to pronounce it however you think it oughtta be.
  

1. Tends to encourage those who are learning or have learned Esperanto to learn Ido in addition, or instead.  Some of those who do that will "defect" to Ido.
2. Allows one to consider Ido as a path to learning Esperanto.  Since Ido has easier grammar than Esperanto (mainly, no plural adjectives and few mandatory accusatives), one can learn it first without worrying as much about grammar, and thereby acquire a knowledge of the international word roots in Ido and the affixes (prefixes and suffixes) that are common to both Esperanto and Ido.  Having learned the vocabulary of Ido, one can then focus on the grammar of Esperanto without worrying as much about vocabulary.  In this way, one can learn both Esperanto and Ido in not much more time than one needs to learn just one of them.  Mind you, even when the vocabulary matches, you'll need to learn about the extra inconsistencies in Esperanto.
   

Learn Ido or Esperanto.

• "Natural" days: they could have numbered the months and the days-of-the-week, thus making their names easy for everyone, but NOOOOO.  Instead they give a special name to each one to please their irrational love for "naturalness" over ease of learning. The weekday names are generally incomprehensible to English speakers, although at least in Ido you'll immediately recognize "saturdio" and "sundio".  Even English is better, in that every weekday ends in "day" so that a learner can immediately tell that an unknown word refers to some kind of day.  No such luck in Ido or Esperanto.  Likewise, the words for "yesterday", "today" and "tomorrow" could have been written logically as compound words meaning "day before this", "this day" and "day after this", which everyone would have readily understood, but instead they chose special words not found in English.
• The month names look more like the English, but you'll have to learn some small differences.  Some examples: januaro = January, marto = March, junio = June, Ido agosto = August (but Augusto in Esperanto), decembro = December.
• Numbers are too damn long.  Who in English would read "1967" as "One thousand nine hundred sixty-seven"?  Almost no one, 'cause it's just too long.  Yet that's what we're expected to do in Ido and Esperanto.   In Esperanto it's "Thousand nine hundred six-ten seven", while in Ido it's even worse: "Thousand AND nine-a-hundred AND six-a-ten AND seven". Absolutely rediculous! They should have developed a system that was quicker, especially for large numbers but also for small ones.  Unfortunately, Esperanto was hindered by its quest for simplicity, and Ido by its quest for "naturalness", which IMO is quite overrated and besides, is there really any natural language in which "1967" is eleven syllables?
• Some ambiguities that do not exist in English.  For example, "port-" means both wear and carry, so if I say "I 'port' a shirt", you can't tell whether I'm wearing a shirt or just carrying it, although listeners will normally assume the former meaning.
• No numberless nouns: If you want to refer to something but don't know whether it is one or more than one, you have to guess, or (if writing) say something like "thing(s)".
  

• Inconsistent relation between meanings of verb, adjective, noun forms
• No word root for "should" or "ought to".  Instead Esperantists have an idiom for "should", "devus", which literally means "would have to".
• Too many "single-word idioms", compound words whose meanings are different than suggested by their component parts.  Examples include "eniro", literally "in-go" which means--can you guess?--"access" (or perhaps it means only "access to an entrance"--but my dictionary sucks.)  If we had to use a compound word for this, something like "enirpovo", "ability to go in", would have been better.  Then there's "aligxilo", literally "to-become-tool" which I, as an Esperantist, would interpret it to mean "tool for getting somewhere".  But in fact, it means "application form".  "Kompreneble" ("understandably") apparently means "Of course", but in that case, how does one distinguish between the different ideas "understandably" and "of course"?  Now, some of these idioms are actually comfortable to Englishmen.  For example, "Cxiukaze", literally "in every case", actually means the same thing as the similar English idiom "in any case": i.e. it's a way to signal "getting the discussion back on track" or "changing the subject".
• Someone wrote some more thoughts here: http://www.kafejo.com/lingvoj/auxlangs/eo/quirks/grape.htm
• Long "is".  In English, the word "is" often uses zero syllables, as in "It's good".  The Esperanto "is", "estas", is two syllables, which annoys me to death.  Ido's "is", "esas", is also two syllables but there's a special exception for this word: You can shorten it to "es" if you want (which I always do.)
  

• Extra long numbers.
• Ido has many changes with respect to Esperanto that seem to offer no benefit, especially to an English-speaker.  For example, Esperanto "sav-", meaning "save", is immediately recognisable to Englishmen, whereas Ido "sav-" means "know" while "spar-" means "save".  Somebody should have told them: if it ain't broke, don't fix it.  But what do I know?  Perhaps in every other language "sav" means "know".
• Overeager acquisition of roots: Whereas the original Esperanto design had 900 roots (eventually ballooning to 9000, and that's just the official ones), Ido started with a full 9000.  Notably, Ido shuns word formation by opposites.  When it comes to very common words like "bad" and "small" and "less", I do agree that a language should have individual, short roots rather than forming these concepts from opposites, as Esperanto does: The words "malbona", "malgranda", and "malpli", meaning "opposite of good", "opposite of large", and "opposite of more", are too long.  However, for less common (albeit not uncommon) pairs, such as friend/enemy, healthy/ill, same/different, and winter/summer, IMO it would be better to use "opposite of friend", "opposite of summer", etc, so that the learner need not learn so many roots.  Mind you, the opposite prefix should not be used if the resulting word is really long.  Anyway, unfortunately, Ido seems to avoid using its opposite prefix like the plague.  And there's synonyms, like "bonega" (very good) and "ecelanta" (excellent).
• Ido does not have a non-number-specific form of the term "you".  You must say "you" singular or "you" plural, which is a bit annoying to learn and a bit problematic if you are writing and don't know to how many people you're speaking (though generally one assumes one.)  Worse, there is a singular and plural form of "who", and of course, one often doesn't know how many people are involved until one hears the answer; however, I think it's normal to default to the singular "who".