Most likely J-31 is up against KAI KF-X, japanese ATD-X[mini F-22].
China will needs to use better Engines, bring changes refine changes to the design specially wings/tail...even the KAI KFX sketches looks impressive.
The Japanese one is a demonstrator. The Americans flat out refused to give engine technology, so the ATD-X uses fourth gen, it's weight is greatly reduced, and it's thrust not up to par, hence two engines. The Japanese is compensating for the absence of weight to thrust. This thing can barely carry itself let along ammunition.
The korean one, I'm not sure the details, but at this point they are even copying the J-20. So their thing is still on the drawing board and I doubt the Americans would give engine to the Koreans. Technologically the Koreans are not really ahead of the Japanese. Though at this point in terms of military, China is more advanced than both.
Korea and Japan are technologically advanced, but you don't go from nothing to engine, no matter how advanced you think you are, currently only the Americans have it, the Russians sort of the Europeans are not even trying. We are testing our version in 2015.
Also they don't have the cash. Especially the Koreans. They wanted a super carrier to go with their 10,000 ton+ DDGs, but as it turns out they got no money. So they are stuck, pretty good, outdated, ship, with no air support.
The Chinese design of J-20 and J31 is fine, all of these designs must be tested before they go into first flight. If it didn't work, it wouldn't have been approved, they are not idiots.
