Speed: Manufacturers tend to make alot of fuss about very small differences in perfomance. Unless you're going to be burning a lot of discs, a 16-speed burner will serve you well. It's certainly not worth paying over the odds to save a few seconds.
Internal or external?: External drives are considerably more expensive than their internal counterparts but are easier to install and can be shared easily between PCs. Most internal drives use a standard EIDE interface, so check that you have a compatible port. Drives fitted with Sata interfaces are a better option, but older PCs won't be compatible.