And why not? In terms of enjoyment, they most certainly are comparable. I can play two totally different games and still enjoy one over the other. If I said I enjoyed Left 4 Dead 2 over Civilization 5, is that an invalid comparison? Because they're not very similar games?
I'm not sure why people get this idea like 2 good games or 2 not-very-similar games aren't comparable. Maybe if he was asking which is the better game for slaying dragons or something; but just asking which one you enjoyed more is a valid comparison.
That being said, I'd prefer Skyrim (or the Elder Scrolls Series, whichever you're referring to). It just seems to have a lot more playablity and overall excitement to it. I haven't played all that much of the Assassin's Creed games, so I suppose I'm biased, but what I have seen and played, it just doesn't seem like it has all of the great content TES/Skyrim has to offer.
ATTENTION EVERYONE: Please don't bash Cueball 2011 for his opinion. There is always one fanboy who decides to take **** on people for merely disagreeing with them, so please, let him have his own opinion.