Floyd by comfortable decision. Mayweather is the exact opposite of Margarito and won't get consitantly hit. Mosley doesn't have the greatest defense and does get hit a lot. No doubt in my mind Mayweather would get a lot of good, clean shots in on Mosley.
If they were to fight in July that would mean Mayweather would not have a tune up, so in that case I would choose Mosley, maybe even by KO. You need to be sharp to fight Mosely, but if it happened later after Floyd has a tune-up I pick him in a UD. Shane stays at range after clinches and even Cotto was able to make him pay for it, Floyd is a shaper puncher and would land alot of potshots and either run or hold. But I would be strongly rooting for Shane.
if it happens, mosley gives mayweather his toughest fight since castillo. mayweather has never fought a pressure fighter with the ability of mosley. and mosley is strong. but mosley does get sloppy at times and can be hit. mayweather wins it by a SD but an upset can definately happen.
PBF by early ko lol but in all honesty i would have to say Floyd but all around scorecards of 116-112 or 115- 113