As a Russian, I have no doubt that China would win. Our military is not in its greatest shape (it wasn't for 30 years), and there are too many Chinese.
China has territorial advantage (attack would come from South Siberia, there are almost no people here), numerical advantage and technological advantage (though it's a maybe, I don't know enough about China's military tech).
But that will ultimately lead to bombs and nukes, so it's a tie at best (or worst)