I actually think Morrowind has a much better world, but Oblivion's gameplay is better. I've said it a million times, but die rolls ruin the authenticity. They're fine in a turn-based game, but when you're standing over a mudcrab, beating the piss out of it with a hammer, and missing every other swing, I just can't suspend my disbelief. I like the physics-based combat of Oblivion so much more.