Here's a great shot from China Defense Blog of China's J-20 stealth fighter alongside Russia's Sukhoi PAK FA and the American F-22 Raptor.
I'm no low-observable design expert, but the undersides of the Raptor and J-20 look a heck of a lot cleaner than the Russian jet. This fits reports that the jet's design trades stealth for increased maneuverability compared to the F-22.
Also, where's the space for the internal weapons bay on the PAK FA; between the engines, forward of what looks like an aft-EW suite? It's been reported that the weapons are carried there and in the two bulbs that can bee seen next to the air intakes.