They both had great defensive ability, and liked their foes to come at them, so they could counter and jab. Of course, this could make this a very dull fight. But still, one would have to come out the winner, and my money would be on Louis. The reason is simple enough - he was better than Holmes in every department!
Holmes had a longer reach and would outweigh Louis by 10-15 lbs in their primes, but he would not dwarf Louis and I can't see the size difference making too much of a difference.
Holmes had a great left jab, one of the best in the history of heavyweight boxing. But Louis' jab was better, or at least at the same level to that of Holmes.
Holmes was a briliant boxer, but Louis is widely recognised as the most flawless heavyweight fighter of all time, so I would give him the nod here also.
Louis had better handspeed than did Holmes and definitely hit harder.
They both had great stamina and heart.
Granted, Holmes had the better chin, but the "chin-factor" would not be a factor in this fight because Holmes would not have the speed or power to knock out Louis.
I don't think Louis could knock out Holmes either, because of Holmes' tremendous chin. But I do see him taking control of the fight in the mid rounds, after about four-five evenfought rounds. He would beat Holmes to the punch and make it impossible for Holmes to slug it out. He would start putting pressure on Holmes (something he hated) and start punishing him with lightening fast combinations to the head and body. Holmes' chin and great defensive abilities would help him survive the full distance, but he would be soundly beaten.
Joe Louis by UD.