Despite being fantasy RPGs, they're very different games.
Witcher 2 is a more focused, story-driven RPG, it also used to be quite tough, but they've apparently made combat easier than when it was first released when they revised a lot in the expanded edition.
Skyrim on the other hand is a much more open world, do what you like sort of affair. It's main strength, especially on PC, are the number and diversity of mods on it, but I and many others I know find, like a lot of Bethesda games, it feels a bit 'empty' as though they did half the work, and then left the polish to the modders.
Still a good game though, but not to the levels it was hyped to (IMO)