I am more of a "performance" kinda shooter. I am underwhelmed by a "rifle cartridge" at subsonic velocities for the sake of lowering the report. A buddy and I have goofed around in some remote areas, shooting suppressed firearms with standard ammo. You can hear the 'crack' but you cannot definitively pinpoint the direction from whence it came. Not without a handful of shots to "triangulate". The gent(s) that offer the 6.8 SPC as a utilitarian alternative have my vote. I built a 6.8 AR with a cheapo Numrich Arms .277 blank. The thing is powerful hog and whitetail medicine. About seven years ago, an acquaintance shot a medium sized cow Elk in Southern Colorado with his 6.8. She was DRT at just under 250 yards. I would not advocate that one make a habit of using a 6.8 SPC on that sized game, but I would call it a "plus" to know the little engine COULD. The mag and bolt difference is a plus to me. Keeps things ordered and properly dedicated. My wife says I am some kind of retentive something due to that penchant. heh
My $0.02
You do realize there is far more to the 300 BLK than subsonic, right? Factory loadings with 90, 110, 120, 125, 147, 150, 208, and 220 grain offerings. The 6.8 does have a decent presence but you are pretty much limited to 90-120gr bullets. Even the 220g in a BLK is capable of ~1250fps. Many loads are very comparable to 7.62x39 loads. Pricewise, the 6.8 and 300 are about the same to shoot. I'm not knocking the 6.8, but I don't think it is heads and shoulders above the 300BLK although I will concede that it has about 7-10% more velocity and the BC should help it on longer range shots.