I thought Norris was more sound mechanically while Nunn was more instinct and reflex based. Some of Nunn's defensive maneuvers would get most guys knocked flat on their asses.
I also say Nunn and it is easy to realize he was a pure boxer and being rangy, loose and a well-schooled boxer, he just outboxed opponents such as Starling, even Toney b4 getting caught, Barkley, couldn't deal with Littles style but I believe he won back the WBA SMW title against Liles... Norris was more of a crisp puncher, boxer who was classy but engaged in wars against Waters, Brown, Jackson and of course hit the deck for that reason, I believe he boxed purely only in the Brown rematch and definitely went to hunt down SRL and tryin to hunt Boudouani, was hunted himself.