I have both games and to be honest Oblivion is the best it never gets boring it probably is the best game on 360 ive played it for quite some time now n theres loads of little side quests to do, and caves to explore, i only play as humans and use sword so i dont really know what the other classes and races are like, only used arrow for last thieves quest. overall id definatly have Oblivion in my collection pluss the shriving isles add on is great.

Mass Effect is still a great game just didnt really match up to Oblivion in my opinion id rent them first if i was you and see what one you prefer.