I personally believe Pac will stop him in the 9th or 10th round. It wont be from a single shot, it will be from an accumulation of punches throughout the fight and Cotto being completely discouraged after the midway point of the fight because he's not going to be able to land shit on Pac and Pac's speed will be too much for cotto. I think it will be either the ref stopping the fight, or his corner, or cotto himself quitting.
Keep in mind I still think there are a lot of different ways this fought can turn out, but it all depends on Pacquiao and how he fights. If he fights smart - bouncing in and out effectively and staying off the ropes - he'll win pretty easily. But if he decides to trade with cotto, which pacquiao sometimes does, it will be a long "fight of the year" type of fight that could go either way.
In the end I think pacquiao's speed both in his legs and his hands will be what wins him the fight. Also, pacquiao isn't a little man anymore, he's grown into his current welterweight body and he punches just as hard as the best of them at 147. If not harder. People underestimate pac and still see him as a little man. He's not little anymore and thats why Oscar & Ricky & Diaz got destroyed so badly, they underestimated not only his speed, but his power!
Oh and when I say he punches as hard or harder than the best at 147 I mean it. There have been a couple people that sparred with margarito and pac and they all say Pac is the harder puncher. The reason is because punching power isn't just about overall strength as it is as much as the technique, speed & timing.