Marciano had a little more craft , more nuance. Jack walks onto something. Rockys skill level was tested against a higher level than Jacks
This is a close fight for sure. I think it boils down to grit and that goes to Rocky. Another poster suggested Dempsey walks into something and I see that as the most likely scenario. Both were absolutely ferocious, Dempsey much more so early. His ferocity wanes over the duration. Marciano is a notch less ferocious but carries the same pace. Marciano's pressure doesn't wane. I think it's very even and if there was a fantasy fight machine and Dempsey ended up winning, I wouldn't be shocked.
Older or not, his victories over Walcott, Moore and Charles are more impressive than Dempsey's rouges gallery, perhaps with the exception of Jack Sharkey.