I think that Fallout 3 is a better game overall. Better graphics, longer, and a great atmosphere. Fable 2 is a very good game too, just a more linear open world game
I havn't even played Fallout 3, and I reckon it's better, based upon what I've seen so far. I don't need to pay it to know that there is a far larger gameplay, detail, graphics and soundtrack. Though Fable 2 is still a pretty good game, it just doesn't have much to com[pare against Fallout 3.