The Steam version gives you 65k coins on account creation and allows you to purchase in-app credits. It's a way to support the game, if you like it. Plus, it runs natively on linux and windows, if you prefer running a native executable rather than via web (for example, the web version may be paused by your browser if you load another tab, thus, disconnecting you).

So in terms of gameplay, zero differences. But you get a more "comfortable" environment and you can support the game if wish to, with the Steam purchase.