Ok, to everyone saying "They're totally comparable", they're not in this context.
In an argument over which is better, you can't compare them to come to an answer.
"Witcher is better because its combat is better".
Its combat isn't better [Though I find it to be more enjoyable]. Its combat is different. You can prefer the style the Witcher goes for in its combat, or the one Skyrim goes for, but you can't say overall "This is the better combat".
This is why they are, in this context, incomparable. You can look at them both and say "This is different", but not "This is better", which is what this thread is, vaguely, asking for.
If someone were to ask you which were the better RPG, Skyrim or TW2, you couldn't give a flat "This one" answer because they are not immediately comparable overall either. You would have to ask "Do you prefer open world questing, or a more fleshed out semi-linear set of quests" among other questions.
No, this does not mean you can't compare the games at all. You can. In this context, you can't do it properly. If it were changed to "Which game has the better open world mechanics", then sure, you could compare the features from each game easily enough and come to a conclusion.
Granted this is mostly OPs fault thanks to a lack of clear direction for the thread, but TW2 vs Skyrim makes it pretty apparent that its asking which one is better. In this case, they are not directly comparable, but which is better is entirely dependent on the tastes of what an individual likes in RPGs, as they are different beasts along those lines.
OT: The Witcher 2 for me, easily.
Combat I find more entertaining and engaging.
The story is better told and IMO more interesting.
The characters are far more fleshed out and alive.
Everywhere you go feels alive [or dead] and has some sort of atmosphere, as opposed to Skyrim where it usually just existed.
Its graphics are far, far better.
Whilst the choices don't necessarily have a ton of consequence, they can lead to a different experience in playing through, giving it IMO more replayability than Skyrim.
Its also less buggy, at least for me, and more polished.