I think it would depend on who was the agressor party. If China invaded Russia the poor state of moral in the red army would improve as soldiers would rally to the defence of their nation. The EU and the US would probably both intervene on Russia's behalf.
The Russian T-90 outclasses the Chinese Type-85 tank, and in the hands of the elite Russian Guards Armored divisions would pose a real threat to the Chinese. Likewise with the BMP-3 to the Chinese Type-90. The Russian airforce is both larger and more technologically advanced that the Chinese Airforce; it is the best upkept branch of the Russian millitary. The Russian pacific fleet while being more advanced than that the Chinese fleet is in a total state of disrepair and the Chinese would probably have an easy time of it. If it came to it, Russia also has the world's largest nuclear arsenal, including very large stockpiles of tactical nukes.
I think that it would be suicide for China to attack Russia, and it would be suicide for Russia to attack China.