initiative and national project, while the other is the SAC/AVIC initiative and privately funded project
Comparison with F-35, it's a conventional design heavily influenced by F-35/F-22. Once the new medium thrust engine is ready,
it'd have thrust close to the F-35, so obviously it's in the same class as the F-35 but using two engines instead of one, it'd have
less space for fuel and internal weapon load. I wonder why PLAAF is not very keen on a F-35 counterpart. 
Additional J-21 pix 
Ever wonder why J-21 was illustrated by magazines with the arrester hook, carrier background, anti-surface (ship) missile
carried on external pylons? Maybe because people were expecting it to find its niche as a carrier based multi-role stealth aircraft? 
Comparison with J-20 
Rough estimated size. 
J-10 and J-20 
Make no mistake, J-20 is the one that will face off with the Raptor, J-21 if it ever enter service with PLAAF will only be the
lower cost complementary multi-role side kick 
J-20 envisioned for the carrier operation in 2020 
When two J-20 were last seen together, it was less than 6 months ago 
Five types of stealth fighters that exist and fly today. Only the F-22 is in service, with F-35 not far off 
Related link: PLAAF pix chain link