Japan, the bombs ended up with many less casualties. The Japanese were arming women and children while preparing for an invasion. I think I have read that an additional 250,000 US soldiers were projected to die with 5-10 million Japanese. Although nobody really knows what would have happened.
I served my mission in Japan - very friendly people. When they decide to do something, they go all out. I think a full-scale invasion would have been much worse.