Only 2 difference that I know of:
- Content-wise, they are the same full game. The premium user only have advantage on early access (free user need to wait roughly 1-2 weeks to get newer update, at least I think we do, heh)
- A scene player at the main menu with "unlock all" (the current free 1.7.0-3 have premium patch, it might be a mistake, but I don't recommend using the gallery before finishing the game)
But, do enjoy the game. If you can, donate to the dev. as this game's story is good.