Really tough call. I rate Williams VERY highly right now. Like, P4P the best out there today, along with Cotto. That said, I think that Nunn is possibly the most underrated MW king of all time, and, barring a freak punch, could.ve beat anyone at the weight on his day. For that reason, I'd probably favour 'Second to' by close decision, although the awkward and hard punching Williams would be a huge test. I wouldn't be prepared to put any money on the outcome, but my head tells me Nunn, by a close, possibly disputed, decision.
Michael Nunn UD. Much more skilled fighter of the 2 and Paul wouldn't have a huge size advantage like he enjoys over everyone he fights.