There is no popup asking for a donation in the steam version. Other than that, no difference. The steam edition costs money, because there is no way of making the payment optional in steam. We couldn't release it absolutely free either, because the steam fee is a large amount of money for a full time student + the publisher wants her dues as you need a company to upload to steam and we don't have one. If you don't want to pay, the game will always be free in itch though.
EDIT: ALSO! There is no Linux version on steam, because steamOS is mysterious and we couldn't get it to work on it