Slay the Spire gives the player many small decisions where Hearthstone’s dungeon run uses a few larger decisions. The big swings are more exciting but ultimately lead to more frustration, as losing one big roll ends your run while you can easily recover from a few bad small rolls.
For deckbuilding in either game, the decision you are making at any given time is about the same: pick one of three. The total amount you are deciding is roughly comparable, a couple dozen cards. Hearthstone groups those into big chunks, where Slay the Spire lets you pick cards individually, or skip if you don’t like those options, or collect gold to shop from larger selection pools, or remove or upgrade cards. You get more options and you feel more in control. I cannot tell you how many times I picked a Hearthstone artifact or card pool and then got almost nothing to go with it.
High stakes are exciting! Talent trees that give you 1% bonuses each level are boring! But small decisions are more impactful when the big decisions are mutable. Adding Arthas to my deck is more exciting than upgrading a card in Slay the Spire, but the choice of whether to pick Arthas is usually overdetermined. Maybe his card pool is clearly the best, or I already have a Cloak of Invisibility so picking a taunt pool is clearly bad… What I am getting at is that there is not much deciding going on, so much as recognizing what works well or badly given what was available/decided previously. You make exactly 11 deckbuilding decisions in a Hearthstone dungeon run, of which maybe 2 or 3 have any tension.
The stack of the deck is another source of randomness. In Hearthstone, you get 1 card per round, and you do not expect to see all your cards in a typical game. If you get your dungeon run artifacts in the first few turns, you win. if they are on the bottom of your deck, they don’t matter at all. You lose to A. F. Kay if you draw the wrong cards. In Slay the Spire, you go through your deck several times per fight. There are lower stakes to the randomization, and you can lower it even further by keeping your deck size small. That shifts importance back towards your decisions, not the luck of the draw.
These two build on each other. That small decision to upgrade a Spire card matters a lot when you play that card several times per fight, versus a few times at all in a Hearthstone dungeon run. Picking your Hearthstone artifacts is the big, exciting decision on getting powerful toys, but the second one happens so late in the dungeon that you use it at most twice and often never, due to the stack of the deck. I upgraded a Spire card just before walking into the final boss fight, and then I played it six times.
High variability with high stakes pushes towards “go big or go home.” But the randomness determines which of those happens.