The second has more to do, more quests, more areas no timer to start with only the games 13 year limit I believe(thats a very long time in fallout games). You'll have a bit more choice then fallout 1 on where you can go, your still limited as in all rpgs by how tough the enemys are in certain areas. This goes for both games though.

Fallout on the other hand has this same 13 year time limit but it has a quest limit of 150 I think days to begin with that you are able to extend another 100 days easily if needed. Once you have done this quest and it can be done fairly early on your free to spend as much time as you like(up to the 13 year limit of course).

So it kind of depends if your intrested in the story or not. I personally would play fallout 1 first even with all those things factored in because its still a great game with lots to do.