The combat in Witcher 3 is much more fun than Skyrim combat IMO. Far more dynamic and it stays challenging through the end of the game. By the end of my Skyrim playthrough I was casually smacking down dragons right and left without bothering to buff, dodge, or think. They say the game levels the monsters but at some point my character far surpassed the ability of the game to keep up with me.
Skyrim crafting ... I dunno. It's kind of addictive in a way and makes you explore more. You can more or less avoid crafting in Witcher 3 except for a few key items, but if you want to survive in Skyrim you have to go dig up materials and craft better arms and armor. Therefore you seek out places to explore for materials, which is good, but you end up dragging around tons of raw materials and dealing with inventory management, constantly dragging your stash of elder ore or what have you back to base so you don't get encumbered while you try to level up your crafting skills.
The storyline and character development of Witcher 3 is exponentially better than Skyrim.
I enjoyed Skyrim but have no desire to play through it again. I do want to play through Witcher 3 again and I need to get the DLCs.