I agree there's more meaningful permutations for replay in Monster Train, and that is indeed excellent. I just feel like there's more randomness vs skill/strategy at play in Monster Train than Slay the Spire. Or again, maybe I'm just not as good at Monster Train? Either way, both games are very good and I'd highly recommend either to almost anyone.
Although I do wish both games gave us just a little bit more narrative/lore into which to couch their excellent gameplay.