It won't matter since Wlad hits hell a lot harder than Rahman or McCall both of whom KOed Lewis with a single shot. And not only did he hit significantly harder than both, but unlike them Wlad had an equaliser in both hands. So, Lewis having a better chin won't matter since Wlad had more than enough power to knock him out cold with either hand if he lands clean. What might matter though is Lewis' superior in-fight adoptability and better inside game. But surely chin won't be a factor for either guy in this fight.