Good to Go Ammo in Texas has a great 6.8 bullet in SSA brass for around .75 apiece for plinking.
The hunting round of 120 grain SST from Hornady is a tremendouse stopper at around 1.10 per round
If you dont wish to reload, there are plenty of people that would buy your once fired brass on 6.8 at .40 cents apiece.
They will pay shipping
So, you could end up shooting the better 6.8 at a net price of .35 for plinking ammo and .60 for hunting ammo
The 300 BLK is nice, but it doesnt come close enough to the 6.8 IMHO
For shots to 200 yards both would be effective. (the 300BLK drops away much faster past 150 yards than the 6.8)
I do think the 300blk beats the 6.8 if you run suppressed, but there are vendors like Bison Armory that are out with 6.8 subsonic uppers
This is a good, fair dialog on the topic, both pro and con:
Good luck on your choices