On Itch, the game is updated to the latest version, but it's under development. You will pay the game price and enjoy all the current updates and future ones when they are available.
On Steam, there are 2 episodes, the first episode is on steam, which is available now, and the 2nd and last one will be available somewhere in Q1 2024, and It will be sold as a DLC.
Itch $15 for the full game (until latest update).
In steam, the overall price will be $15 for episode 1 and episode 2 DLC.
Eventually, both versions will be exactly the same and will be priced the same too.