Agreed that it's an open question. I come down on the side of 'fewer Japanese died' (leaving the question of allied casualties - the deciding factor for Truman - out of it). The Japanese military was training peasants to fight allied soldiers with sharpened sticks (AKA spears). Spears plus overwhelming force and rapid advance were potentially effective until the advent of rapid-fire weapons (e.g. gatling gun, Zulu wars). That was a near run thing for the brits, but these peasants hadn't been trained from youth so it would have been waves of them advancing and dying. And given the bloodbath on the allied side I have to wonder how the following peace would have turned out. At best, a continuation of the racial animus (something that did not happen with the Germans) so firing squads and mass famine. As a kid (early 60s), I worked (volunteer work) alongside veterans of the Pacific campaign. They were only then beginning to admit that Japanese were indeed human beings. I was shocked at the time, but having dug into the war, I get it.