Frazier had better defense, faster hands, was bigger, IMO stronger, better footwork, was just as tough and had just as much stamina. I think they would both be able to hurt the other with their best punches, but I think Frazier would be able to land his best punch more often and to greater effect than Marciano would. It would be an absolutely brutal war of attrition, but I pick Frazier by 8th round stoppage after a bloodied and battered Marciano is dropped. He makes the 10 count but the ref stops the fight anyway before Smokin Joe can continue his onslaught.