Skyrim. I haven't played The Witcher because it doesn't look that interesting aside from the sidequests. The combat looks good for what it is but, seems to have less variety than Skyrim.
Skyrim is a better game for roleplaying and exploration while the Witcher is better for storytelling.
Although i'm not sure it's a fair comparison since one was made with better hardware and is a bit more modern while the other feels a bit dated in some ways.







