I've thought of this one too many times. I've over analyzed it so much that I've had both fighters winning. Today, I think Pryor wins in a close fight. He was just too relentless. Marquez would have plenty of opportunities to counter, and he always seemed to fire back when he was hurt - sometimes too much. I think that would be his downfall and the referee stops it.
Pryor outworks him for a decision. I don’t know if Marquez has enough power at the weight to really make Pryor respect him.
Marquez's best fighting weight was featherweight/ super featherweight, in fact its where the vast majority of his fights were held in his great career. Really, he only went up in weight to face Mayweather at welterweight because of the big money he received. Call it cashing in at the tail end of his career. Of course Mayweather knew he was getting a past prime "name" opponent at a weight advantage that seriously favored him. It's the reason he fought Marquez. He wasn't a threat at that point of his career. Mayweather also had a stylistic advantage over Marquez. Marquez would have the same issues multiplied by two against a prime version of Aaron Pryor. And wouldn't survive him. Pryor's relentless aggression, funky angles of attack, speed and power makes Pacquiao look like a novice. Plus the size advantages, and excellent ability to absorb devastating power (Johnson, Montilla, Cervantes, Arguello, Kamada, were all much better punchers at Jr. Welterweight than Marquez) I'd bet Marquez at the time he faced Mayweather Jr. Or when he entered the Welterweight area wouldn't get past eight rounds if Pryor was in the opposite corner. Hell , Mayweather Jr. Wouldn't beat Pryor at his best at Jr.welter. He wouldn't get within a mile of a boxing ring against him either. He'd know he could lose.
You can’t discount that Pryor wasnt a actually a big guy, so in terms of size this would resemble a fight like Marquez against Katsidis. Marquez wasn’t especially dominant against volume punchers, so I think Pryor would probably throw enough to win a close decision, but Marquez would also have a chance to change the direction of the fight with a perfectly placed right hand and Pryor would present that opportunity.