Does the FC-31 need to match the J-20 in every aspect to be a worthwhile investment? All that is required is for it to offer some capabilities that the J-20 does not bear or to fill a niche (dimensional, financial, logistical) not touched upon by the J-20.
For instance, the F-35 offers specific advantages over the F-22 (including a smaller size, all-aspect EODAS, HMD, EOTS, and a presumably more powerful AN/AGP81 radar), but it would be controversial to claim the former as superior to the latter.
F-35 is actually half a generation ahead of the F-22, and while it lacked F-22's kinematic performance, in most other way it is technically more ambitious and more advanced.
J-20 looks to me like an effort to combine advances made with F-35 with an airframe capable of close to F-22's kinematic performance. But J-31 does not look to me like it is half a generation ahead of J-20. If anything, it looks a much more conservative product than the J-20. It seems more like a speculative venture by the manufacturer to poach low to medium end foreign sales opportunity, and not at all backed by PLAAF.