I like both games, but I favor Fallout 3, it has better combat, beter voice acting, and if you don't count the Metro station, you can say that Fallout 3 has alot more variety in it's locations, unlike Oblivion, it has more copy-pasted caves, ruins, and forts, like the Alyeids all have the same taste in housing.
The only apprecible thing about Oblivion that I think why people would favor it is the HUGE amount of quests. It's like Bethesta already figured people would complain about the voice acting and samey environments and decided the best way to compensate is to load it with enough quests to possibly shame a whole year-round D&D campaign. I guess It's selling point is that you never run out of things to do in Cyrodiil, I just wish the real world nowadays has people giving you quests and rewarding you for your success, would compensate for a high unemployment rate, even if it might have to mean fully legalizing weapons and an increase in body count.