I enjoyed and have completed both GTA V and Witcher 3.
The biggest differences outside of the main story is that GTA V is just doing random chaos, messing around. The side quests, and random activities in Witcher 3 are much more fleshed out. There are proper quests and random encounters that have their own story lines and what you do in these have a real impact on the world, it's much more than just messing around.
Yes it's fun to blow everything up, rob stores, go to the military base, you basically have to make your own fun in GTA V. Personally I could not do that for more than a few hours at a time. I could easily get lost in the Witcher 3 (outside of the main story) for a long time, and I would also be able to see my actions have an effect on the game world.
I personally think Witcher 3 is the ground breaking game and will be a becnmark for future RPG games. I don't feel that way about GTA V, but that may partly be because I have been playing Witcher 3 recently so it's fresh in mind, I haven't played GTA V for a while, despite owning it on PC.
They are very different games. They are both good. Get both