They need to have a balance. In fact, canard do increase RCS by Chinese aeronautic study. But Chinese design also conduct study that with careful all area of treatment and precise flight control. The RCS increase of canard can be managed to the minimal.Does stealth is more important for SEALTH JET or anything else like thrust or agility, let wait and see what will TVC tech uses by J-20 for WS-15 engine, if STEALHNESS is more important for PLAAF than it will go to 2D TVC, if not than PLAAF will go for 3D TVC
Why would Chinese go for canard layout for J-20 even canard do contribute a certain portion of RCS? It offer the agility that the Chinese military top brass require. High AOA is not only for top fight but evade air to air missile by bleeding them with evasive tactic if you detect early.
The 3D thrust vector may offer more option for J-20 compare to the benefit of a 2D nozzle.
Russian is 3D vector but it's not nozzle petal control of the Chinese WS-10 TVC but instead more of angle nozzle control to achieve the change of thrust.No, Russia is using 2D vector, instead of 3D vector.