Given that there is a LOT more variety in the design and look of the side dungeons than there was in Oblivion, I would say that Skyrim is much better. Once you'd seen one cave/fort/ruin in Oblivion, you'd seen them all. Didn't stop me from looting every one I came across anyway, but y'know... the variety is nice this time around 
Sure. I still think the caves in Oblivion were one of the best parts of the game. Given, I only played it for a month or so...
