Fallout, all the way. I HATE Elder Scrolls. The most boring, derivative, story, setting and characters imaginable; an incredibly lazily structured set of quests where you basically just choose a questline from the start and do that, rather than actually unlocking stuff as you go along; the graphics, much as I've heard people praise them, still look absolutely terrible to me; and of course, the horrible, horrible combat.
Fallout on the other hand has the vastly more interesting and colourful post-apocalyptic setting, with the whole alternate history thing going on, and while I still don't think the writing's particularly great, the setting alone is enough to keep me interested. Then there's the fact that these games, especially New Vegas, are extremely well-structured in terms of your progression. You really can't just cut across the wasteland safely, so you have to follow the road the game puts you on, and encounter all the odd side-quests along the way, and there's always several ways to do each quest you're given, and it's just far less lazy than the design on the Elder Scrolls games is. In terms of graphics they're pretty similar, but the style and setting, again, of Fallout makes the still unimpressive graphics look interesting to me - wandering around the wasteland can actually be quite a beautiful experience. And the combat, well, it's still not my favourite but the whole VATS system is fun, and guns feel a lot better than swords to me from a first-person perspective.
So yeah, Fallout. TES is fucking horrible.
