The reason why the Chinese bought into the Su-27 is simple. It is the best available to the Chinese. In 1996, no european country could sell to China due to the TianAnMen. The J-8 upgrade programme was stalled. US had similar restrictions to the Europeans.
Even then, the Chinese could only get the Su-27SK model (which doesn't fire the R-77). It was only later in this decade that the Su-30MK models became available to the Chinese. That's why 170 out of the 270+ sukhois that the Chinese have are still the older version sukhois.
The Chinese actually got the Su-30MK at the start of this decade.
That's why if one reads the 2007 China military report to congress (drafted by the DOD), we'd see that China is finally license producing the Su-27SMK version and we'd probably see upgrades of its earlier Su-27SKs to this version.
The report was probably done by some clerk who should have known better, given the other errors circulating in the report.
China isn't license producing the Su-27SKM, the Russians have not confirmed this. The Su-27SMK is quite different from the J-11B, and both planes are well documented. Despite being very different inside, the J-11B still physically copies the Su-27SK down to the RWR housings near the engine intake.
On the other hand, the Su-27SKM, has offset IFR probe, offset IRST, 12 hardpoints, and don't show the RWR blades anymore.
Inside the planes are very different.
Su-27SKM uses twist cassegrain N001VEP radar from the Su-30MK2. The J-11B uses slotted array KLJ-4, which is related to the KLJ-3 from the J-10. The SKM uses Russian AAMs like R-77, the J-11B uses Chinese AAMs like PL-8 and PL-12. The former uses Series 3 AL-31F for the engine, latter the WS-10B, adding a total of 1400kg of additional thrust. The cockpit of the SKM uses 3 MFDs; the cockpit is the copy of the Su-30MK2. The J-11B cockpit has four MFDs, and a large wide angle wide screen 3D holographic HUD. When it comes to defensive measures, the J-11B boasts optical missile approach warning systems matched to the RWR and all interconnected to the flares/chaff system. The SKM uses the Pastel RWR from the Su-30MKK. The SKM still uses the old Su-27 analog flight control system, the J-11B uses a digital FBW.
To put it in another way, the J-11B uses a lot of J-10 related technologies. One can better put it as a Flanker with the soul of the J-10. The SKM is an Su-27 with the soul of an Su-30MKK.
The Su-27SMK (note the difference of the letter arrangement from SKM), is a defunct project in the nineties that planned for the use of the Zhuk-27 radar, offset IFR and IRST, 12 hardpoints and provisions for external fuel tanks. From the avionics point of view, its very different from the SKM.
Many analysts believe that the J11B is a 2 seater chinese copy of the J-11A. For myself, I think it is just a licensed SU-27 UBK version upgraded to SMK standard that received a lot of Russian inputs to develop. With all the troubles the Chinese went through to get the Speys, the WS10A is probably another licensed production (although Chinese propaganda will continue to suggest that it is a "chinese-made" engine. China's tech is not that advanced and many are prone to over-estimate their capabilities.
Analysts are full of crap. Starting with, Chinese designation do not use "B" to designate two seater, which is US parlance. The correct designation for Chinese two seater J-11 would have been either J-11S the S being the Chinese word for twin, or JJ-11, if its really just for training. They did not call the two seater J-10 JJ-10 but J-10S, which means the two seater J-10 is more than just a trainer and has unique capabilities of its own. That does not make the J-10S just a two seater version of the J-10.
The Su-27UBK is not licensed to China by any means. But making a twin seater isn't something hard for China to do. After all, the Chinese actually made twin seater MiG-17s and MiG-19s when the Russians never actually made such planes (only planned it). China never got the plans for the twin seater MiG-21F either, before their relationship with the Soviet Union turned cold, so they pretty much reinvented the concept for their own JJ-7.