I'd pick McCallum for sure. He'd have a size and strength advantage in my opinion, and would work nicely off Napoles' offense. Initially, Napoles might win rounds and even seem in control, but I think McCallum gets better and better at timing Napoles and making him pay.
McCallum wins on size. Napoles was the better fighter. Napoles would almost certainly be considered the greatest junior welter of all time had he stayed at the weight. He was the ideal 140 pounder. Napoles whups Pryor, Benitez, Floyd, and Chavez at 140. And he'd wipe his ass with Tszyu.