I love and adore them both, but they don't honestly seem like separate games to me. it's almost like they're mutual expansion packs with different storylines. Taken separately, Fallout is the better game from a technical standpoint - it's cohesive, intelligent, and driven. but Fallout 2 is my favorite simply because it's more of what I like in an RPG - it's sprawling, hilarious, and varied.
I recently revved up Fallout for the first time in a couple years. I wanted to play Fallout 2, but my brother took it upon himself to delete the game off his computer, and when I went to reinstall it, the disc would no longer work (GRRRRRR.) the excellence of Fallout shouldn't be underrated, but after about a week I had beaten it and there was literally nothing else to do. it's such a SMALL world compared to Fallout 2, and while it lends it more focus, it also feels constricting to me. There's less of everything, and while some people prefer a smaller environment, I'm more of a crowd person.
There are some things Fallout does better, though. The Glow and the Brotherhood, for one. I still get shivers when I think of the Glow. the first time I went there, many many years ago, I got FRIED.