I personally think that it will be an exciting fight. Although Mayweather has been in some not so fan friendly fights, I think Cotto will bring out the best in him. Cotto will pressure him, try to land first, and always have a punchers' chance if he connects cleanly. Mayweather, on the other hand, will block his punches with his defensive wizzardy and counterpunch cleanly at every opportunity he gets. Just as he did with Mosley, I also see Mayweather going on the offensive if he gets control of the fight to a point where the punishment may warrant the knock out.
It would be great to see Cotto spring the upset but I think Money will beat him more easily than he beat Oscar. The 1st 6 rounds will be competitive IMO - after that Floyd will be in command.