Oblivion captured my attention for longer probably that Fallout for the simple fact that i had ran all up and through Oblivion before i played Fallout, so i basically had a better idea of where they were going with much of the content in FO. Therefore it didn't take me as long to complete everything.
One thing i like more about Oblivion is that it took significantly longer to complete JUST the story mode. I started Fallout, got out of the vault, did my own thing for about 10 hours of gameplay, and decided to do a few main quest objectives. My goal wasn't to finish the game, cuz i wanted to do that after the majority of the side quests were done, but by the time i got a few quests into the story, it was over. That was very disheartening to me.
One thing i like more about Fallout is that the combat system was fucking awesome. Fighting enemies rarely became tedious, and VATS, need i say more?
Oh and on a side note...Neither oblivion or Fallout 3 were inherently First-Person. In both games u can change the view mode to 3rd person, and in Fallout on the PC (after the baby shit and all that), it started me out in 3rd person. Playing in 3rd person brought on one major problem, and that is of picking up small items in a clutter around other items u may not desire to pick up or items you may not have wanted to access, but i prefer 3rd person for both of those games. Sure in Oblivion the fighting looks a little "wonky", but that view mode seems to lend to higher production value to me.