If you purchase the current version here on itch.io, you will get access to all future updates here (if there are ever going to be any).
As far as I understand Patreon's purchase mechanic, it will be the same over there.
From my own experience, if you're a Patreon supporter, you will get access to the game download on Patreon for as long as you are a paying supporter. After that you won't have access to the download.
The versions are identical on Patreon and itch.io.