Ali's is much more proven - in my book Marciano avoided a lot of punishment a lot more successfully than certainly the 1971 onwards Ali - hence we never saw Marciano absorb the kind of beatings that Ali soaked up - for me we can only really go on what we've seen so for me Ali's is much better - he certainly proved he could ship huge amounts of big shots that Marciano never had to take - saw Marciano take big shots against Walcott, Charles and Moore - and some digs from Layne too but with Rock it was usually him dishing out the pain
marcino was down twice ali 3 times[you can't count the wepner kd he was stepping on ali's foot at the time] yea ali had a better chin i think
Ali fought Foreman, Shavers, Lyle, Frazier...even if Rocky stands full distance with Liston he doesn't beat that.
Both men's chin were made of granite. Ali's was the more proven one,as he met harder bangers in his career,than Rocky did.