My two cents
Airframe / aerodynamics: The Russians have a massive head start, and they build the most manoeuvrable, beautiful fighter jets in the world. The Chinese experience is based on F-7 copies (J-7, JF-17), or designs based on other jets (J-11, J-10). Advantage Russia
Engine: same as above, even more gap in favour of Russians, the Chinese are still trying to copy last-gen Russian engines and engine tech is not something you can steal as easily, you need the Industrial base. Huge advantage Russia
Radar/avionics: The Chinese would lag in this again, and from an Indian perspective doesnt matter. The Indian PAK-FA will be like the Su-30MKI, marrying western electronics with a great airframe. Advantage Russia, huge advantage Indian version
Stealth: here China and Russia are starting from scratch, so its a matter of who can spend more on R&D (China) and steal tech easily (China). But joining hands with India, and the financing India provides, reduces the gap. So small advantage China
Hence, we would have J-XX being slightly more stealthy than PAK-FA, but the gap would be very low compared to F-22 vs other jets today. In most case, therefore it will boil down to closer range traditional combat, where the PAK-FAs superior engine, aerodynamics and (in case of the Indian version) avionics would prevail, probably a 2-to-1 kill ratio.