I also say TKO, with Cotto no doubt beating Margarito to the punch in every exchange the punishment will be too visible and officials are forced to stop it, I'd say more like around 9-11.
Margo by late round TKO. He is just too tough, big and strong. Cotto will slowly find out that he cannot keep Margo off of him and the pressure applied by Margo will break Cotto down.