Huge differences. The biggest being that one is an island, and the other a peninsula. A peninsula which we already have military bases on.
Yea, we did not need the bombs to eliminate Japan. Just so long as we were willing to accept 2+ million friendly casualties, and 14+ million Japanese casualties.
It would not be all that high. Consider the Korean War.
That war started in June 1950, and by July the US and South Korean forces were pushed into a little pocket, the Pusan Perimeter. Then in August they pushed their way out, until by October they had 2/3 of the North Korean Army in POW camps, and controlled the entire peninsula. At that time it was a short war, and the casualties were actually fairly light.
Then on 25 October, China invaded. That caused it to drag out for another 3 years and caused horrendous casualties.
But the high casualties were not caused by the North Koreans (force strength 266 thousand), but by the Chinese (force strength 1.3 million).
Personally, I do not think the Chinese would get involved again.