Soviet invasion of japan WW2?

If the Japanese didn't surrender is it reasonable to assume the Soviet Army would have done their own version of Operation Downfall? After their success from Operation August Storm in Manchuria and northern Korea would they have continued into Japan, making a US invasion of Japan unnecessary? If we had not dropped the atomic bombs, would Japan and the USSR slugged it out? Would that be good for the interests of the U.S? Since the U.S was fearful of Stalin and wanted to end the war with Japan, would that be killing two birds with one stone? In the end, who do you think will survive USSR or Japan?