Longtime Hearthstone and Gwent player here. Hearthstone is far more polished and overall, I'd argue, the much better game. It's stale, sure, but whenever an expansion releases, you can have fun for a solid 1-2 months.

Gwent has crazy potential, but the shabby UI, rickety technical foundation and insanely unimaginative card mechanics make it a drag to play at the moment, at least for me. The potential remains untapped as of now.

I'd suggest you wait for the release of Gwent and then ask the same question again. As it stands, I can't recommend it to anyone, much less propose that you should spend any of your money on the game.

(I know I'll get downvoted to hell for saying this.)