If the Russians attacked the Chinese with the intention of seizing the whole of China, without using Nuclear, Biological, or Chemical weapons, then the Russians would not win, though they would probably make a few "most kills" records and the prestige of being the only power with the balls to test the People's Army of China on its home turf. Here, Russian technology and their skill at anti-partisan operations would result in massive Chinese casualties (not all military mind you), numbering in the tens of millions, while Russian losses would remain in the region of a few hundred thousand. The biggest issue would be public support at home for the Russians, since it is always hard to convince people that starting a war is a good thing but if there is one thing that I think would cement the Russian resolve however, it would be their visceral hatred of things Oriental and particularly Chinese. However, they would not lose either, since the only thing that would prevent total victory would be the fact they could not seize all of China, though what they did seize would count for a lot. It would be a long bloody war with no clear winner, though the price would be much heavier for China than Russia.
However, China invading Russia would be much more interesting because the terrain favours attack north, not south, and the Russians simply do not have anything along the length of the Chinese border in regards to major population centres, except near the major ports. However, the Russians always maintained a large force along the border, so any Chinese attack would be hard-pressed to advance far before suffering under Soviet air-power and tanks, as well as the fact that the Russians don't have to carry their supplies with them so much when defending. Again, the casualties would be massive for China while comparatively low for Russia, but weight of numbers would count for something since you can't be strong everywhere on a border thousands of kilometres long. China would eventually find a weak spot and pour a few hundred thousand men through it, turn a Russian flank and bingo, the China are marching into the hinterlands of Russia. One small thing... What the hell is worth taking in this area of Russia? The rail-network is small, the distances from town to town being dozens if not hundreds of kilometres, and worst of all, by the time you got far enough west to threaten European Russia, the Russians' would have brought millions of its best troops forming an impenetrable line from the Urals to the Caspian or Black Sea. It would be a reverse Stalingrad for China to try and breach this wall. Germany had a hell of a time trying to supply its much smaller armies in WW2 over the distances of Ukraine and the Caucasus and China would be trying to supply much bigger armies over vast stretches of land several times those distances. There is nothing to forage in the area, there is little more than vast grasslands flowing farther than the eye can see. Chinese numbers wouldn't mean much if they lost thousands every week to starvation and lack of supplies. Worst of all, invading Russian land would inspire that most fearsome of Russia's weapons, the Narod (People). The people of Russian have an instinctive dislike for the Asian peoples and to be invaded by them is sticking a pole in a killer-bee colony. The Russians would fanatically defend their positions and it would not be unlikely that Russia would employ Chemical and Biological weapons to even the odds, especially using the latter amongst civilian populations in China to decimate the Chinese's source of manpower and supplies. A Small-pox epidemic in China could kill 200,000,000 in only a few months if deployed in the right spots. The only places where China could really hope to make trouble is near the ports, like Vladivostock. Russian troops would be hard to beat but a few million concentrated Chinese troops could break through and besiege the city quickly. However, the Russians would be able to supply it by sea, as the Chinese Navy is no match for the Russian Navy and if there is one thing you can count on, it is that Russian marines and naval personnel are amongst the finest warriors in the world at that time. Between their naval supply and their Marines fanatical defense, Vladivostock could have become a Stalingrad for China and sapped the will of the Chinese Army.
Of course, just to make it easier, Ill do a quick compare on their abilities
Navy- Russia in all aspects. Training, technology, discipline, and quality of production all beat China's fleet.
Army- China has the numbers, the willingness to fight in both offence and defence, their infantry are skilled at infiltration which means any Russian defence would have to be very well laid to counter this, and the simple fact that they are literally more likely to run out of guns and equipment before they run out of soldiers.
Russia would have the edge in the defence, as they need real incentive to really give it their all in the offence, but their good training and solid values would ensure they didn't fall apart. Plus, that visceral hatred of all things Mongoloid would make a Russian more likely to kill a Chinese soldier than an American should he be faced with them.
Air-force- Russia. The Red Falcons of the Soviet Air force were a force to be reckoned with, masters of the dogfight, and planes made to be rugged and able to work in just about any place or condition. Russians could rely on thousands of planes to help supply their forces. It is ok to say that the Chinese would be roughly equivalent to the Russians in 1941 when compared to the Luftwaffe. Chinese numbers would mean little once they meet Soviet SAMs and fighters designed to tackle vastly superior western planes. Chinese Air defences are weaker, using almost all Soviet deigned equipment, which means Russians could easily use ECM to render impotent the Chinese SAMs as they know exactly what frequencies the Chinese radars would be using. Russia would also never send airborne troops without a massive escort to ensure fighters don't intercept. A few SAMs would succeed but not nearly enough to seriously undermine the Soviet airborne assault's abilities. Russian philosophy is based on making sure that any unit can operate at near 100% efficiency with up to 15% losses, which mean a division of 10,000 is able to do the same job even with 1,500 casualties, which on the heavy side for an airborne assault before it lands.
Tanks: The best Chinese tank was a T-54 derivative that never really exceeded the original in anything, not even numbers. The Soviets could have brushed aside with ease any Chinese armour that foolishly wandered into the open. The Chinese tanks would have been useful in the defensive, where the rugged terrain of the Russian-Chinese border makes for good ambush, but a T-72 could quickly make scrap out of any Type59 it encountered. In short, Russia had better tanks, more of them, with better crews, better training, better doctrine, and more reliable fuel supplies. Russian MICV and APC simply blow away their Chinese competition.
Infantry weapons- On the light infantry level, China was usually a couple of decades behind Russia in infantry weapons, as well as inferior skills in the use of weapons like mortars and light cannon. Russian Artillery simply was better in all aspects to Chinese though they sometimes had the same guns. Russian ammunition is superior (Chinese ammo is shitty to say the least) and the quality of their equipment's production is higher as well. While China retained the type 56 (a knock off of the early AK-47), the Russians continued to improve the gun, creating the AKM and AK-74.
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