I know I get a Steam code if I buy on itch, so for me there's no difference between purchasing from one platform or the other. But do you get a bigger cut of the sale price from either site? I would prefer to give you my $$ as much as possible!
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I like to buy on Itch just because I like their management more (letting creators choose what percentage of sales to give, promoting interesting free games pretty actively, etc.). Steam is always tempting because of cloud saves, though...
ETA: Which is why I'd like to say thank you for including the Steam key, so we don't really have to choose in this case!