For story, characters and world I'd have to go with Fallout. However, for longevity I'd go with TES. Fallout's cool, but there's just not as much to do (for me at least) as with TES games. There's no where near as many weapons, equips or item types and generally there's more quests in TES. There's also a greater enemy variety in TES and the locations are a lot nicer to traverse. Barren wasteland is cool and all, but after 10+ hours of gameplay it pretty much all looks the same.
I do love Fallout, and I get much more excited about Fallout's releases than TES' (and I really hope this new 'project' is a Fallout one!) but they seem so short lived compared to TES. I mean, I'm still playing Oblivion how long after it's release?
Fallout is my nice clean sneakers that I like to wear when I go out drinking, but that hurts my heel after a few hours wear. TES is my comfortable, familiar pair of old slippers that I can lounge about the house in all day long.
I have a good friend who really likes Fallout (3 and NV) but who laughs at me for playing TES games because it has "mages and shit" and he really doesn't get that when it boils down to it, they're both pretty similar.