Well, thanks for bringing that question up ; I don't play Gwent myself, I have tried it through TW3 and it wasn't my cup of tea, so to speak (my reason is, I felt like the game was too simple in appearance, + didn't seem that mindgame was important). But I know that many people out there love the game and will tell me wrong (I hope so). I would like to compare both games like which is the more complex or strategic, I don't know honestly. But I can tell a bit for MtG.
MtG Arena is pretty much like any MtG : it's feels good and nerdy, and the more you get access to cards, the more you know how the game plays, the more you make strategies and decks, the better, in my opinion. Many MtG fans despise MtG Arena for being a "virtual" collection game, so you lose the biggest interest which is collecting real stuff.
To me, MtG is good because to be real, I never had many people around me playing the game, nor having enough money to spend in buying cards, so I left the "material" game to play on the "virtual" one. And the best about MtG Arena is to be able to read all the cards, so you can make your builds more efficiently ; there is an intelligent deck building system (and so far, not pay-to-win) ; + online play 24/7 so you can test them infinitely with humans (some bots around here too, it happens). Finally, what I like with deck building is there are not supposedly "best" builds, they may be some strategies that are more favorable, still there are just too many strategies, so a single deck cannot counter all of them. The mind game is still present like in Poker, even if you don't see the people (the mind game can occur, e.g. : in keeping a dummy card to make believe that you have a big card to play).
The little "plus" is the game modes and challenges they put everyday, that make the players try new things. And if I'm right, this game is free to play !
Now, there are also downsides on it too, like finding players that play 200 card-decks which is absurd, or overabuse of the Planeswalker cards which makes me roll eyes because I know that I'll lose whatever it takes, because I don't play these cards myself (I find these cards too powerful). So yeah, MtG Arena is worth trying as it's free, maybe it lacks some polishing and some extra-rules specially for the online play, but still you can have tons of hours of fun building your decks and planning strategies. Oh, and you can write in your friends profile number and challenge them (instead of matching against random trolls).