From what they told me on Discord- and I have also heard this from other devs- itch allows creators to keep a higher percentage of the total revenue. Also, they have Creator Day sales where devs get 100% of the profit! It's a great time to grab something off your itch wishlist.
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It's a complex issue, and each dev will have their own priorities, so it's great to take the time to ask the devs you care to support.
In our case, Itch and Humble are probably the stores the most aligned with our company values, but at the end of the day, we believe each player should decide what's best for them, because it's your hard-earned money. In practice, the crushing majority of PC gamers still buy on Steam when they have a choice. So while some other stores are more generous with royalties, devs rarely see a huge impact on their bottom line because relatively few people buy on Itch vs. Steam.
Meanwhile, it is true that Itch allows devs to decide how much of the royalties on each sale go to Itch. In our case, we believe in Itch's mission, so we set it so that they keep the same percentage of royalties as Steam does.
Have a great day!