Skyrim wins hands-down on many fronts: lore, quest design, world/dungeon design, immersion, and particularly npc's. Out of the box, it's a remarkable game. It's only in terms of longevity and roleplaying that I feel it gives ground to Oblivion ... and to me, that's a big thing. I have spent thousands of hours in Cyrodiil, with upwards to 20 different characters--imagination is the only limit, because a single PC could do literally anything ... even completely ignore any and all major quests. Skyrim does not allow that freedom.
Pick one guild that suits your character's abilities and moral outlook, and I guarantee you'll run up against a situation where you must do something that completely goes against the grain, whether it's sneaking around in heavy armor (with a war axe strapped to your back no less), stealing or murdering someone in cold blood ... there's no other way to complete the quest. And if you choose to do none of the main or guild quests at all, some areas are inaccessible (you can't get to anything that's inside, even if it's only from a minor sidequest) unless you join the faction it's tied to. That rankles.
I foresee probably three character types in my Skyrim future, and then I'll go back to playing my Oblivion characters. And that's a shame, because I really like and am enjoying Skyrim, and I
want to like it a lot more. But at present it's just a nice place to visit--modded or not, I really doubt I'm going to live there.