Fallout had several things going for it that Fallout 2 didn't. Fallout has more unique things in it, and I think they were stretching the engine beyond its limits in certain aspects, but the results were good enough. Fallout 2 feels subtly more predictable in most areas since most of the scripts are more generic. In Fallout, you never knew what someone was going to do. In Fallout 2, however, they staid well within the bounds of what the engine could do well, in most cases.
Fallout 2 does fell more polished, though, and better in an RPG-aspect, inasmuch as there are many more things to make your character interesting, like perks, surgeries, etc. You can level up to your wildest dreams (Anyone actually *get* to level 99???). The Evil path is available, really, if not very easy. I hope I'm making sense here...