I really do not know where the surname Rubin comes from. However, There is a Rubin in my community who is a Cohen. He claims that the name Rubin comes from the Ruby stone on the Choshen that was worn by the Cohen Gadol (High Priest), hence the origin of him being a Cohen.
As far as I know though, I am not a Cohen.
Rubin and Rabin could essentially be same name different pronunciation . Derived from the word Rabbi