Well, as they are both hugely popular, both are also from PR, usually they are being compared in some aspects of their game. That being said, who's got the meaner left hook? Tito has long left hook which is really his bread and butter punch. Cotto can take his to the body with devastating effect. I guess it's still tito as it has saved him a lot of times and has spelled doom for a lot of guys. a tito left hook ended the reign of maurice blocker, started Vargas' dilemma, took out william joppy, lastly, it took away daivd reid's lead on the cards.
Trinidad could drastically change the momentum of the fight with one shot, and his best punch was the left hook upstairs. Tito had a good left hook to the body too, but Cotto is more consistent with it to the body, and ultimately does more damage.