Russia and China share a long border, and there will always be some security concerns because of it. India does not share a border with Russia and has no territorial disputes, and thus they get better treatment. You see Russia entering co-development projects with India, selling the Indian Navy aircraft carriers, and even leasing a nuclear powered submarine.Originally Posted by Gollevainen
China is important to Russia's defense industry today, simply because the Chinese have more money to spend. But Russia is not selling their best stuff to China and would prefer to keep a "quality gap" for their own security needs. Also, from past experience, China tends to absorb imported technology to domestic production. So from long term view, the Russians have to think "well we're selling this stuff to them now, but in 10 years they might be producing better copies and selling for less".