But I don't think that's the reason Russia prefers China.  I suspect Russia sees China as a better potential ally than Japan.  Many are expecting China to be the next superpower.  Militarily and economically.  

During the 1930s, when Japan was heavily engaged in its imperialist expansionary adventure in China, Russia found it necessary to maintain a very large troop presense along the border between Manchuria and the far eastern parts of Siberia, as well as in Mongolia (then a de facto protectorate of the USSR).  For a very long time,Japan coveted the resource-rich areas of Siberia and had dreams of slicing off a nice chuck for itself.

It is not too widely known that in October 1939 Russia and Japan came to blows in the Mongolian desert in a place called Nomanhan. The Russians came out ahead this time around. This was no minor border skirmish, as the Japanese had something like 20,000 casualties and the Russians roughly 15,000. While the pressure on Russia eased after the Japanese started fighting the US in the Pacific, it was still necessary for Russia to maintain many divisions in the area in spite of their being badly needed on the Eastern front against the Germans.

In fact, the Japanese Army and Navy were bitter rivals: the Army favored attacking Russia to split off  a piece of Siberia, while the Navy was in favor of taking control of the Pacific to get at the resources of Southeast Asia.

So, the relations between the two countries was never good and probably still isn't.  

They seem to think the old Russia and China vs. the U.S. dynamic is alive and well.  I think there's something to that.  Japan has no real military, and they're in our pocket, anyway.  If I were Russia, I'd court China, too.

Now the talking heads are lamenting the failure of "free trade."  Opening up to China was supposed to tame them by showing them the benefits of trade.  Instead, it's had the opposite effect.  They are getting more aggressive, not less.