In the past week, there have been a lot of speculations regarding a deal where Russia would export 24 Su-35s and 4 Ladas to China.  Now, there have been official denials from Russian government and sukhoi has also not put this on their website.  That would indicate this deal is definitely not done yet.  I normally would ignore these su-35 rumours from Russia, but there have been enough support reading through the Chinese sources for me to think that there are serious discussions for this.  In addition, Chinese sources also indicate that there could also be sale of S-400 and IL-476 as part of a large deal.  Obviously, this would be the largest sale package from China to Russia since probably 2002.  I will just look at the individual parts of the deal and whether they make sense from a Chinese point of view.
Back in 2008 when the su-35 rumours first came out, it made sense for China to buy 2 or more regiments of Su-35.  As time went on, it seemed like the domestic flankers produced by SAC have been more or less satisfactory for PLA.  I often read on Western/Russian news sources about how they are shocked to see Russia is still willing to sell such an advanced aircraft to China even after China “cloned” Russian fighters, but those articles really do not seem to have a good grasp on reality.  We know that China has two “stealth” fighter jet programs under development that will probably achieve IOC sometimes toward the end of this decade, so it doesn’t make sense for China to buy and then “copy” a large number of su-35s.  Shenyang AC is actively developing and producing naval and fighter bomber versions of flankers in J-15 and J-16.  Su-35 is mostly an air superiority aircraft, so it’s not going to help those projects.  At the same time, China is also not exporting any of its flankers to other countries, so this export deal will not threaten Russia’s other export markets.
One of the reported reasons from Chinese side for purchasing Su-35 is the coming end of production of J-11B.  They have requirement for 1 regiment (24 aircraft) of air superiority version of flankers before the more advanced 5th generation fighters can enter service.  While that is possible, I think su-35 will create a logistical problem in the future like the Sov destroyers with the Chinese navy.  They will need to maintain a new type of aircraft, a new engine, a new generation of Russian avionics and Russian missiles.  That would seem to be a lot of trouble for just one regiment.  That would lead to my conclusion that they are purchasing this strictly to get their hands on the 117S engine.  Russia made it clear to China early on that they would only be willing to sell 117S to China as part of a Su-35 order.  I think 24 is probably the minimum number of Su-35s that Russia would be willing to sell to China to allow Chinese access to 117S engine.  China does have the largest MRO plant for AL-31F outside of Russia.  All maintenance work for AL-31 is done inside China.  I would assume 117S maintenance and life extension work would also be done there.  Despite improvements in the reliability of WS-10A, I still read about problems found in deployment.  If there is one problem that can cause real delay in J-20, it would be not having a reliable engine solution in its development and early deployment.  117S would also be possible options for J-10 and J-15/16 projects.  If China does choose to purchase Su-35s, access to 117S engine would be the primary motivation.  And Russia would benefit by exporting su-35 and possibly large numbers of 117S engine later.
The deal for 4 Lada submarines is more interesting.  Many PLAN followers have asked why China would be interested in purchasing so many units of a submarine that Russia has not even accepted into its own service (its AIP system is not going to be ready until later).  Typically, China chooses to only purchase mature systems that it can quickly induct into service.  On top of that, it seems to some that Chinese submarines seem to have reached the technology levels of their Russian counterpart with that mysterious new conventional submarine in 2011.  The reality is that Chinese submarines still have a way to go in stealth.
As part of this deal, China will be getting transfer of technology along with local production for 2 of the 4 submarines.  On top of that, some unreported Chinese subsystems will be going onto these submarines.  So, the question is what is China providing as part of this submarine and what is it interested in?  The currently mass produced 039B submarine are already equipped with AIP system that can be installed on the Chinese version of Lada submarine.  On top of that, China could install its own sonar system and combat systems onto Lada.  The latter part should not be surprising since China has also installed its own sonar on kilo submarines.  What China seeks as part of this deal is the Russian’s design of Lada submarine and its noise insulation technology.  From Lada, China could learn how to design and build a single hull submarine with conformal sonar.  If we look at China’s new submarine that came out in 2011, it seemed to adapt numerous features from Lada submarine.  This submarine is supposedly one of a kind built to replace the old Gulf class ballistic missile test bed.  It is probably too large and expensive to be mass produced.  Based on their experience from this submarine and Lada project, it’s quite possible that China’s next series of submarine would look somewhere in between (possibly single hull) and have many of the features currently on Lada class.  Russia also has a lot to gain here, because it needs a new submarine for export to replace kilo.  Regardless of whether Russia chooses to use any Chinese subsystems for Lada in the future, Chinese involvement in this project will ensure that the export version of Lada becomes fully developed and commercially viable.
So while I’ve read numerous panic articles online about how this deal will significantly improve China’s capabilities and shift the cross strait balance of power even more toward China’s favour, I think those articles really show very little appreciation of China’s current military industrial complex.  While these purchases will help and speed up PLA development, they are by no means game changers.  However, these purchases will improve ties between the two countries at a time where Russia constantly complains about the trade imbalance.