It would have been a staggering disaster for either side to attempt war against the other after the defeat of Germany. I put the matter slightly in favor of the Soviets because, as others have noted, their officers in the field by the end of the war were generally superior and were supremely committed to following orders. I suspect the stalemate line would have fallen somewhere west of Berlin and held there. Scandanavia would certainly have been a lost cause to the Allies, eventually leading to a serious threat of an invasion of Britain and most probably the signing of an unfavorable long term truce.

No significant eastern ground invasion of the Soviet Union would have been attempted, whether from occupied Japan, China, or across the Pacific. It's foolhardy. Minor conflicts might have gone the Allies' way, but taking a strip of useless coastline is meaningless.

I take for granted that nuclear weapons would not be a part of such a conflict because of the political and sociological consequences. The potential further use of nukes against Japan after the first two was even a matter of debate and, obviously, subject to the personal approval of Truman. If nuclear weapons were authorized for use they would be decisive in favor of the US and allies because the Soviets, IMHO, simply could not have reliably guarded their population centers from the sheer number of angles of attack that were possible against them.