From what I read 6.5 would be better at the longer ranges.
I stopped at Midwestern Shooters Supply in Lomira, WI today, they had 2 choices of 6.8 on the shelf. Zero choices of 6.5 on the shelf.
If there had been some Hornady 6.8 SST's on the shelf, I would have bought some.
Hornady, Remington and Silver State Armory are the three most common brands of 6.8 ammo.
Also saw a couple of .300 BLK choices on the shelf, one standard velocity and one subsonic.
For 6.5, better plan on rolling your own.