Will this game come with a Steam key?
No, if you purchase Sickness on Itch, it does not come with a Steam key.
However, Sickness is currently on sale on Steam for the same amount as here, so if you'd rather the Steam version, you can still buy it there.
is there any difference between the steam version and the one here on itch.io
Sorry, I never got notified of this reply.
For the sake of anybody checking this thread, the answer is no. The game's content, be it on Steam or Itch, does not differ at all.