Typically, the Fighter starts off strong, and just gets stronger. The Wizard starts off pretty weak, and gets a LOT stronger. Fighters are simple to use and can be fairly tough to master. Wizards are intermediate to use well (not just use, because anyone CAN use a Wizard, but idiots can't use them too well), and can be pretty tough to master, since you have to choose spells carefully and take risks or minimize risks.
First, all the stuff on the Fighter. It's tough, it's powerful, it's got a lot of physical prowess and feats. They can use any armor, shields and two-thirds of all weapon types with one level. Good attack, good defensive options, good Fortitude to resist those damned death-spells, and of course bonus feats that make them versatile.
The average (average meaning recommended abiltiy scores) level20 Fighter has a +15Fortitude bonus. This isn't too bad. Then you hit the +7Reflex and +6Will, and find you've got some pretty glaring weaknesses. NWN "fixes" this by giving things like "Immunity: Mind-affecting Spells" and "Improved Evasion" as equipment enhancements. This is bullshit. A level10Fighter/30Weapon Master weilding a scythe with those two enhancements becomes immune to basically everything. The only way to beat that is to have Immunity: Critical Hits, and high enough resist that he deals decent damage, not excessive. Good luck. Fighters aren't supposed to have those things. They just aren't. They aren't meant to be dodgy, or impossible to stun. They're supposed to be the muscle that smashes. If you want those enhancements, but don't want to multiclass, Party up! Wizards need some meat shields sometimes. In all truth, Epic Fighter is meant to be partied anyway. All epics are. What happens when said Weapon Master meets Emsu, when Emsu is in full form and power, which means 100%immunity to bludgeoning/piercing/slashing damages? He gets his ass handed to him.
Fighters have a LOT of options to multiclass and spread their skills out. They don't HAVE to be just a muscle. I have seen Dexterity-based Fighters and them I don't mind giving Evasion-like items to if they can prove they deserve it. That means you generally multiclass, though. Fighter/Rogue is pretty neat, try it. For that matter, Fighter/Monk is one of the most powerful natural builds there are. Multiclassing with Fighter is generally helpful, unless you're trying to focus in spellcasting of course. A Cleric/Fighter can be pretty deadly, and Paladin/Fighter might seem a little redundant, but it really isn't. If you have an Epic caster with auto-still spell, even Fighter class there can be a good move.
Second, the Wizard stuff. It's weak, it's unable to continuously cast for an extended period of time without items to use, it doesn't have an overabundance of feats. It can't use armor or shields, and only gets the crossbow, staff, dagger and club weapons. Low attack, low defense, low fortitude and barely enough hit points to survive.
The average level20 Wizard has +8Fortitude, +8Reflex, and +13Will. Saves aren't great. Until you add that they gain +1Universal against spells for every 5Spellcraft ranks (with item enhancements). Thus, that same character, using 23Spellcraft ranks, actually has +12/12/17 to spells, and if you add the spell Protection from Spells, +20/20/25. So basically a Wizard has awesome saves against another Wizard, or the spell-like abilities of many monsters. Even higher if you work at it.
But of course, saves aren't everything. The Wizard has horrible physical power and accuracy. ... So what? The Wizard can also cast things like Greater Ruin that deal an average of 123 damage in a single strike, or Maximized Isaac's Greater Missile Storm, which deals 12x20 damage at once. And yes, that's not 240 for a reason: Since they deal 20 separate attacks of 2d6, even a 5/Magical resist can greatly reduce the effect. For instance of that Maximized, it reduces from 240 to 140 against 5Resistance. 5 becomes 100. Every spell has some sort of weakness to it if you look. This is why Wizard is tough to use: You have to work at getting a good spell selection for anytime, or take time to reselect for certain areas.
This problem is mainly NWN-based, due to the easy ways to increase saves: The DC for spells is often low. The DC for spells is 10+spell level+relevant ability modifier+focus feats. This means that, since the highest possible base ability for spellcaster scores is 38 (18+10 from levels+10 from Great feats), coming to 50 with enhancements, even with maximum focus the highest DC you can get is 36+spell level. So the highest DC is 45 for 9th-level spells and 40 for epic spells, as they can't be focused in. That seems high and all, but a level20 Monk/Paladin can have universal saves in the 30's. At level40 they'd be insane. Or even if they multiclassed to Monk/Paladin/Blackguard at level20 it could be insane, because those Divine Grace and Dark Blessing feats stack.
In fact, I'll point it out for you. 18Monk/1Paladin/1Blackguard means base saves of +15/11/11 before any modifiers. If you have say 20Charisma after enhancements, these improve to +25/21/21 before other modifiers. Add the feats for them and +27/23/23. Go further and say you have 20Dex, 20Con and 20Wis after enhancements, and they become +32/28/28. Add a Cloak of Resistance +5 and +37/33/33. Seeing it yet? That means a level20 character has a 65%/45%/45% chance to save against a DC45 spell from a level40 Wizard. Further, the character has a 90%/70%/70% chance to save against said Wizard's EPIC spells. By level30, the DC45 save%'s are the ones from the epics before, and the epic saves are all at 95% success. Is that right? Hell no. But it's possible, if you work for it. There isn't enough for Epic Casters to empower themselves in DC. This is why so many depend on spells like the missile storms and Ice Storm, because they don't allow saves.
Looking more generally now, a Fighter tends to start strong and grow in a pretty steady fashion. A Wizard, on the other hand, starts off weak, but grows exponentially. To illustrate this I even took the time to make this:
Fighter
Wizard