Watson, he has the power to hurt Barkley, and the chin to take his best. He's also the much better boxer, his high-guard keeps him safe whilst his short shots up the middle keep Barkley either honest, or over. Side note: I can't believe Watson's career is the one being questioned. He literally outclassed the guy who KOed Barkley in one.
I don't think you can use this fight as a basis of how a Barkley Watson fight would go. We all know Tommy hated swarmers and at this stage in his career, he did not have the energy or stamina to keep Barkley at bay. What was telling was that when Hearns opened up, all too rarely, he was on a completely different level to Barkley and had Tommy been a few years younger, like in the first fight, Iran would not be able to live with Hearns, getting overcocky aside obviously. Michael Watson was a really good, solid, technical fighter, strong and with a decent dig and chin. A big middleweight as well - he'd be strong enough to out-hustle Barkely and with a much superior skillset, he'd wear Barkley down and perhaps stop him late rounds.
Benn improved vastly after the Watson fight he fired his trainers and went to train in America. Hearns high guard didn`t work against Barkley as well as it did v Hill.