They both have a tendency to gas in the mid to late rounds. I favor Cotto because of his body punching, but like the cliche says, speed kills and Berto has a hell of an uppercut. A good fight that I'd love to see happen.
I see great potential in Berto of course, it's difficult to put it in words but it's kinda tryin to outspeed a sneaky shooter like Zab, Cotto couldn't do it but he overwhelmed him and busted him up in the end... Cotto would test that chin and Berto wouldn't pass a 12-3 minutes rounds test.