Fallout 3 is an excellent game but I enjoyed Oblivion a lot more.
So between the two I'd recommend Oblivion instead, the Game of the Year edition which includes both expansion packs. The Shivering Isles expansion pack alone is quite big, almost like a separate game of it's own.
The problem with Fallout 3 for me was mostly the drab depressing atmosphere, which I guess is the effect that apocalyptic wasteland tends to have on people.
The fantasy atmosphere of Oblivion was a lot more fun and exciting, as well as all the sub-quests and there are so many ways to play the game, for instance you could be a holy knight, thief, mage and even vampire just to name some.
So it has a lot more replay value and I'll even probably play it again sometime.
The nect game on my list will probably be Resident Evil 5...