Hello! Buying here on itchio doesn't give a Steam key, unfortunately. I'm hoping to release it on GOG in the near future, but nothing definite yet! The Steam version has the standard amount of DRM, maybe? I actually don't know how it works with them, whatever the default is -- I didn't add any special DRM to the Steam version.
The Steam version has 26 Steam achievements that will only work in that version, I think that's the only difference!