Most likely MM based on feats. Mysterio has some sneaky good stuff, but when you're a Flash villain, it's easier for even average showings to be higher-level. I'd LOVE to see the fight, though.
Mysterio's a likable character, too. The original, at least. He's all about originality and being clever; his scheming is more about the challenge than to assert some kind of megalomaniacal dominance like many villains. Always enjoyed when he and Chameleon teamed up. They had a cool perspective about everything that the more "serious" villains never seem to, not unlike the self-aware camaraderie shared by the Flash Rogues at times.