itch.io

Nothing fundamentally, steam's a little easier to keep up to date also has achievements and a few other steam related things. Itch gives more to the dev. Patreon gives you extra perks however is a continuing investment in the future of the game.

Note buying on itch also gives you a steam key, Patreon also gives you one with the correct levels. 

Hello and thank you for the question! As the other person said, the only real differences are in the form of Achievements and Cloud Saving. Though it should be noted that any purchase on Itch will also get you a Steam key.