As per China, J-20 is more fighter bomber, have u seen the massive size of the aircraft its like two J10s, hence very easy to detect and locked on, it has more range and weapons space, but not an ideal platform for dogfights, for which u need low radar feasibility and better avionics sensors with increased speed.
We donot need so many missiles in our FC-31 P, limited number but definitely more than JF-17 will suffice, the best for defending Pak Skies.
Have u ever wondered y most modern world air forces 'prefer' to have small sized or medium sized aircrafts. Even the F-22 is smaller than J-20. it is overall capability and efficiency of aircrafts, notice Russian fighters are big and hence larges RCS and long turnaround times. The Russian big fighters have lost more in numbers, as compared to western or american jets of medium and small size, which hardly any shot down.
J-31 / FC-31 P fits the PAF bill perfectly as smaller size, its budgeted requirements, stealth qualities, fuel consumption, low maintenance, mission requirements and similarity to JF-17 as a new upgrade.