I did give a reason, but you probably didn't see it cause I edited my post. The settings the two games put you in are very different. It probably depends more on the person playing the games than the games themselves. I prefer the relaxing, and nice feel that Oblivion gave. Whereas Fallout 3 was a bit more lonely and depressing. I'm not saying don't play Fallout 3. I'm saying play Fallout 3, then play Oblivion.
And you get most mods at
www.fallout3nexus.com/ for Fallout 3.