Hardest puncher vs hardest chin. McCallum was more measured and not as spectacular as McClellan (other than the highlight reel of him vs Julian jackson). If not for the spector of Nigel Benn on McClellans record, I'd vote overwhelmingly for Gerald. the Nigel Benn battle did exist and as a result, gotta go with McCallum on this one.
They had different styles of defense. Benn was very awkward and difficult to touch, he was head-first and low down, evasive and very unpredictable... whereas McCallum was very good at parrying, with less dramatic rolls and effective footwork, or would just counter below the shoulders.