Having played both Hearthstone and Gwent, I'd say Gwent is so much better in every aspect, and it will take over this "card game genre" pretty soon.
Some pro Hearthstone players are already switching games, even.

Playing Gwent just feels so much more satisfying and rewarding than playing HS, and I'm not even a Witcher fan (I've only played about ~5 hours of The Witcher 3).