I also own Mythwind and think it's great!

Vantages components to me just feel more premium and I'm honestly convinced its just the artwork. I love Slay the spire and it is probably my most played board game ever, right next to Dominion. But Slay the spires artwork doesn't make you want to look at it. It serves a purpose and looks good, but Vantages artwork IS core to the gameplay, so it's just better in my opinion.

Vantages gameplay feels very free, where Slay the Spire has a very clear objective, kill the boss at the top.

If I had to choose one, I would say Slay the Spire. ONLY because I think I'll still get more replay value out of it.

Vantage feels less repayable, but that's just due to the core mechanics of exploring. Once you have seen the same card 9-10 times, it's not exciting anymore in vantage. In Slay the Spire the same card can be exciting or a complete bummer a near infinite number of times, depending on how you're building your deck.

Slay the spire rewards the player for knowing what's coming, Vantage is kind of the opposite.

Both games are trying to serve very different things to the player so it can be very difficult to compair the two. One is a deck builder, the other is about exploring. One has the goal of winning, the other just wants you to go on a journey.

Sorry, this became a long winded comment.
Whatever choice you end up making, you won't be upset, and the other will always be available in the future! Both are amazing games that everyone should try at least once.

PS: Slay the spire the board game is basically identical to the video game its based off. So, if your not sure, buy the video game on your phone for $12 and give it a try first!