Are you more interested in open-world dungeon exploration or a plot-driven more-linear RPG?
Which appeals to you more, a nordic snowy mountain isolated style, or a gritty medieval down to earth style?
Some people are probably going to have strong opinions one way or the other (I'd expect strongly worded opinions saying the Witcher 2, and condemnding Skyrim
), but I would suggest it depends on your personal preferences. Also, correct me if I'm wrong, but I seem to recall the Witcher 2 having a lot more melee combat, while Skyrim has archery and magic as options.