I'm going to say Hearns on the basis of objectivity- 100% of his fights 147 and under were won under the distance. There is a good chance this one would too. I wouldn't put it past it however that many people idiotically have Hearns KO1 God at 147- I won't say he's overrated because he isn't but some take this to the extreme by looking only at his right hand, KO victories, and physique.. To bring things back down to earth, his welterweight title reign lasted only a handful of fights, lasting roughly only a year until he lost to another ATG in Leonard. Every fighter is human and has his flaws. Ultimately you never know- I wouldn't put it past it if Pac (or any solid hitter) cracked that chin.
If Pacquiao lasted six rounds I'd be very impressed. He's not really a ''full'' welterweight really, whereas Tommy was more than comfortable up at 175. It's a bit like asking if Marquez could beat Duran at 135. Not happening.
Manny was using the same gameplan for the Oscar fight. Moving in and out and staying off the ropes. Roach was afraid of Margarito's size, so he had Manny do that all night long. That's the same thing he'd do to Hearns. Really Hearns can win this 2 ways. He can knock Manny out early or he can try to box with and knock him out late or go to UD. When I saw Hearns move around and out box Leonard, my mouth just dropped. Pacquiao has no chance