Everything has it's trade offs.
The pluses for the 300 AAC Blackout are many, for the most part its just a regular AR/M4 with a different barrel that is in .30 cal. The negatives are the limitations (good old Issac and his laws) of pressures and mass. But these work well with sub sonic loads. (I don't do SBR or suppressed and still like the 300)
The 6.8 SPC side steps some of the limitations (a little) with a combination of pressures being exerted on a smaller area.
The 6.5 G does this even better.
The negatives for both the 6.8 SPC and 6.5 G include but are not limited to; magazines, bolts, source brass.
If I were rich and wanted to maintain my AR lower and was going to hunt in the wide open spaces, like Montana or Wyoming or well you get the idea, I most likely would go for the 6.8 SPC.
If I were rich and wanted to poke holes in targets at 1000 yards and use an AR lower, I would go with the 6.5 G.
This does not mean that either can't do the job. And for that matter the 300 Blk could do it too but not by me.
If I want the best for long range hunting, I would not limit myself with an AR lower. Like wise, if I want to poke holes at very long ranges, an AR lower would not be my first choice.
But to dispatch big pigs at ranges like I have around home, any of the three would do it very well. And the 300 BlackOut is not as taxing in the process.
One of these days I will find my final load for my 300 BlackOut and move on. Maybe a 6.8 SPC or 6.5 G or ?