There was a thread on here previously, but it's about 4 years old. Technologies and designs change, so...
I actually haven't purchased a 300 BLK upper, yet, but more on that in a minute.
At this point I'm not really concerned about sub vs super, just reliable feed and function. As far as accuracy goes, I'm going to look at the 300 like I would a pistol caliber carbine. Something fun, cheap, and easy to load for, and gets me decent accuracy at 50 (or maybe even 100) yards. I have zero desire for the expense or effort of gas check, so I'm planning on PC'ing whatever I end up with, and shooting it at velocities that make it friendly to do so. Would prefer a 4+ cavity mold, but you can't have it all. So edjumicate me!
Regarding an upper... Pretty much the same criteria I've set for the mold. Doesn't need to be a world-beater in accuracy, just want to get something that's reliable and affordable. PSA seems very spotty on their availability. MAS Defense seems like a pretty good candidate ( http://www.shop.masdefense.com/300-AAC-Blackout_c3.htm ), but I don't know anything them?