Who should win? I don't think Jones had trouble against southpaws. (minus tarver, not prime then)However Michael Nunn was fast and 6'3 (6'2?). I don't think Jones fought someone that tall and fast at middleweight. Nunn was supposed to fight Jones at some point but lost the chance.
I would never go against Roy vs. most guys and he would beat Nunn. Nunn was great in 1988 when he beat Tate, but he was never a Roy Jones Jr. caliber. Roy would win a close UD.
It's actually quite an interesting matchup prime for prime, as much as I think Nunn gets overrated on the whole. Also, a fight between the two wasn't in the works until both were fighting as Lt. Heavyweights, by which point Nunn was well past his prime and likely wouldn't have made much more of an impression than the other oldies Johnson and McCallum.