Ali's was more proven, although Ali was dropped, hurt, and staggered more often than Marciano was. And Marciano's style was to come forward which makes you come into the power. I would still say Ali's pretty definitively, although Marciano was hurt much less. Ali did face some of the bigger punchers in the history of the division so we have to give him all the fair credit in the world.
You could argue it either way. I would go with Ali based on the data at hand because his chin was more tested.
both was about as good as you can get. many consider that alis chin was never entirely tested until his second coming yet he still had the sonny banks, henry cooper knockdowns and was KO'd by kent green as an amatuer. marciano had none of those experiences earlier on but was physicaly more mature by the time he was hit hard by the beter level fighters. but then rocky would have faded badly had he continued as long as ali at the top level. ali had the best jaw of any veteran fighter IMO but was that because he was hit less when younger? marcinao never EVEN made it to a veteran fighter. ali survived frazier, foreman and shavers best bombs but also rode shots beter. marciano also took flush finishing blows from moore and walcott who scored chilling knockouts. both were 10 out of 10 for durability what ever way you look at it.
There is simply no way to prove Marciano's chin was in Ali's league as a heavyweight. None. Ali's was as proven as a chin can possibly be, that I'm aware of.
Ali, far more battle tested. A far longer career against a bigger variety of fighters as well. Also, before anyone cites the Cooper fight Marciano was hurt just as badly against nearly no name opposition early in his career.
Marciano was knocked down, glassey eyed, rubbery legged and was lucky to be saved by the bell early in his career? Wow, that's news to me.
Holding on for dear life like newspaper reports have Marciano doing might not be 100% as bad, but I've heard he was certainly in a bad way. Have to figure however that Cooper was actually a serious banger. The man who had Marciano in trouble was not.
It's quite a lot to say because it's not a film. We don't know how fabricated or exaggerated the journalism was considering the writing style in those days. Everyone seems to point to Marciano fights that aren't on film as a reference to his durability/chin which I find very amusing. I'm sure it's all over-stated.
As a percentage of fights against contenders I'm not sure, Ali fought far more top contenders Anyway chin isn't a constant
If chins are best tested when they are hit by hard punchers ( my judgement on them ), Ali should rate higher.
I would go with Ali. Both had great chins, but Ali's was more proven and longevity has to be a factor too. You also have to consider the style differences between the two. Rock was much more planted and had a lower centre of gravity, where as Ali was very often on his toes and moving. Both proven at the top level though, guys like Moore and Walcott were very dangerous punchers, Rock was a very durable guy.