I went with the 6.8 spc over the 300 BLK for my dedicated AR hunting rifle, and I really believe the 6.8 is better suited for that role. The 6.8 is a faster, harder hitting round than the 300 blk in all bullet weights (or SD's) for which they are comparable. Even in the extreme upper end of factory 6.8 bullet weights, the 140 gr SSA load is still booking along at 2400 fps. While bullet drop can, of course, be accounted for, most people seem to prefer flatter shooting rounds that simplify the bullet drop memorization/calculations required for normal hunting distances.
The load I'm going to use for deer this year sends a 110gr Accubond out of the muzzle at 2660 fps, which should be about perfect for our small southern white tail.
I do think the 300 BLK is a fine caliber, and I will most assuredly be buying an upper when I get around to picking up a 30 cal can, its just the 6.8 is better suited to the hunting niche.
Looking back though, it looks like the OP intends to mostly use this AR for 3 gun competitions, if so, why are you even looking at the 6.8 and 300? The only round that really makes sense for AR shooting games is the .223.
p.s.~ If you decide to look at the 6.8, I would check out AR Performance and Bison Armory first, they are pretty much the go-to 6.8 makers.