York Arms is building my upper in .300Blackout (#12) right now; I've heard of most of the others in one form or another... but I hadn't heard about the 7.62x40WT (#13). From Wilson's page:
The cartridge is designed for 110-150gr .308” diameter bullets loaded to supersonic velocities, but if chambered in a 1-8 twist barrel is also suitable for heavy bullet subsonic use also.It's got just a hair more ass to it than the 300BLK; factory loads are showing 125gr at 2350fps (up from 2200 or so)... Aside from that last 100-200fps, I don't see much in the way of advantage over the 300. Doubly so since (to my knowledge), the WT isn't a SAAMI-spec round - so it's all subject to licensing from Wilson, and therefore more expensive.
Still - I'm glad to see people trying new things.