Does Marciano really hit harder? It's debatable. Dempsey KO'ed Fred Fulton, a huge 6'5" guy and a respected contender (I believe he was ranked #1 at the time) in 18 seconds. The job he did on Willard is still shocking, it seems unnatural to see a 180 lb fighter send a 250 lb man staggering around the ring like a ragdoll.
In terms of punching power, Dempsey is top ten all time material. I think Marciano may be as well, but you are selling Dempsey's punch short.
Bodypunching and inside fighting go to Dempsey. He had a method to his bodypunching, he threw short shovel hooks and uppercuts in close quarters. He threw them in combinations. You can see this in the Gibbons fight. Marciano on the other hand, a bit like Tyson, needed some space for his punches. He couldn't knock a guy out with 6" punches like Dempsey did.
The only qualities I give to Marciano for certain are stamina and chin. Footwork, punching technique, natural reflexes, handspeed all go to Dempsey.
Marciano always has a chance with "Suzie Q", but I think Dempsey stops him in the middle rounds, or perhaps even with an early assault.