Honestly, It seems like both China and Russia will be reduced to a bunch of glowing craters when this is over with.
China ... probably. Russia, I'm not so sure. Stalin was, if anything, not a risk taker. Well, not one to take greater risks than absolutely necessary. Provoking an atomic power with strike capability against Russia's heartland (against which he can't retaliate fully) would probably be too much for him. He only ever got involved if he was guaranteed a shot at victory (either real or, as the Winter War demonstrated, perceived due to failures in intelligence and analysis). It would be severely out of character for Stalin to do anything more risky.
Now, that's no guarantee some twat in the field does something incredibly dumb, but it won't be a political decision.
If anything, Stalin's likely to exploit this opportunity to subdue China and curb the CCP's independence while making noise about capitalist atrocities. The Americans carpet nuking China is a win-win for Stalin, really.