It depends on your preferences, I think.

Fallout 3 is the better exploration game, New Vegas is the better RPG. Now, I love Fallout 3 and I think it has the best world design in the series (lore not included), but I get a great deal more enjoyment from leveling a character toward a specialization and seeing the different ways my decisions affect the world than I do from dungeon crawling.

New Vegas has me covered there, its perks are really fun and almost all of its many quests have multiple solutions or an alternative quest. However, if I want to scavenge through the wreckage of a dead world I can think of no finer game than Fallout 3.