For me Fallout 1 is better than Fallout 2 because it's more structured, it's something that they carefully crafted to progress along nicely, it didn't have more than it needed and it didn't have too little. Fallout 2 on the other hand has tons of content, way more than Fallout 1 but it doesn't make it better. Fallout 2 can be a mess to play through with its main story because it feels like it's put in the background of all the side-quests. Simply put, it's easier to get lost in Fallout 2 and not in the good way.
Fallout 2 allows you a lot of opportunity to roleplay different things but it loses sight of what is important, which is the main quest. In Fallout 1 there is always this constant quest pushing you forward: Find the water chip and later on destroy the mutant threat. In Fallout 2 on the other hand once Hakunin's messages are over and Arroyo is demolished by the Enclave it sort of stops. Yeah you need to find your tribe and save them but that urgency to find them kind of diminishes when there isn't anything to connect you too it anymore. One day you're out to save the tribe the next you're in a boxing ring fighting Mike Tyson. One day you witness an elder die in front of you as your village burn and the next day you're digging graves outside of Reno. Once that final message by Hakunin goes through your kind of on your own and no matter how long you wait to complete the main quest there is no urgency to complete it, you can lollygag around and do whatev's.
Hell, there isn't even all that much dialogue with other NPC's about it, some of the GECK dialogue is just jokes. Finding the Water Chip was urgent and it shows with the dialogue, finding a GECK on the other hand is like a wild goose chase where every time you think you are near it it ends comically (drunk in Klamath passing out, Renesco getting pissed off, stuff like that) the storyline just doesn't take itself seriously enough outside of Arroyo to feel like it matters. From what I remember in Fallout 1 whenever there was hints of a water chip it actually lead somewhere and whenever it was talked about the protagonist was more serious about his quest to save his vault.
However, while I think Fallout 1 is better it doesn't change the fact that Fallout 2 has more stuff to do in it. I can't become a pornstar or get fucked by a super mutant in Fallout 1, so when it comes to just mindless fun in a good cRPG I think Fallout 2 takes the cake, but as an art-piece I think Fallout 1 does a much better job.