No comparison is perfect but Fulton seems, by descriptions, to be Fulton seemed a bit more nimble with serious snapping power... and a weaker mandible.
There was talk of Dempsey to come out of retirement when Sharkey won the championship. Dempsey was interviewed and stated he would like to fight Sharkey especially since he could not take it to the body. Once Carnera beat Sharkey there was again talk of Dempsey coming out of retirement to fight Primo. Of course in his prime Dempsey destroys both fighters. Those that watched both Dempsey and Marciano fight did not rate Rocky over Jack. “You cannot compare Marciano to Dempsey except in terms of punching power”. (Fleischer).
Dempsey is the superior fighter imo, and was still thought of as so when Marciano ascended the throne.
Primo will spoil Jacks advances with his MUCH superior strength in the clinch IF he gets close Jack is four inches shorter standing upright and six or seven crouching he is also several weight classes lighter then Primo in his prime all muscle as well! With no plastered gloves I see this being similar to the Gene Tunney fight a boxing clinic Jack pressing the action and being befuddled when Primo flurries on him and dances out to safety behind his ridiculous reach accompanied with his ATG foot work. He has the most knockouts of any heavyweight champion for a reason if you knew your history you'd understand Dempsey was a protected cheat and Primo the "Beast" was actually gentle and TOOK dives for the mercy of those in the ring with him.