Mayweather. Mosley at LW had either equal or greater speed and Mayweather has shown he knows how to deal with a faster fighter. PBF was the epitome of composure while Mosley was frustratable as evidenced in both Wright fights and the 2nd Forrest fight. There wouldn't be a KO considering both fighters also had phenomenal chins, but i see Mayweather taking less damage and doing a little more to get the win.