The WT can use the standard bolt, the X39 must use a non-standard bolt opened up to the limit of the milspec material strength. Because of that, the x39 bolts made from 9310 are preferred.
The magazines have an even bigger influence. Wilson is machining the rib out of aftermarket poly mags, but in essence they are the ones specifically designed to operate in the AR15 straight feed mag well. For the x39, the cartridge taper forces a curved mag design with cutaway mag well. Works great in the AK, but the frankenmags required to fit into an AR straight mag well become notoriously problematic, and good ones are hard to source.
Given x39 shooters aren't really getting ammo any cheaper than 5.56 shooters, preferring it becomes a matter of accepting the compromises in engineering to get a power increase. Since the market has better alternatives, like WT, 6.8SPC, 6.5G, and others, the x39 really drops down the list in not only ballistic terms, but reliability operating out of the AR15.
It's a total package, not necessarily a Chinese menu.
