If you handload and need a 800+ yard rifle Grendel.
If you don't handload 6.8 and don't have to have an edge at 600+ yards and up.
If you are sick of paying out the wazoo for 6.8 or 6.5 brass and reload.
1. Buy 7.62x39 brass case reloadable ammo.
2. Shoot it out of your choice of firearm that doesn't mar the brass too much.
(Skip step 1 and 2 by buying empty 7.62x39 brass for reloading, the only new equipment you will need are AK/AR franken mags)
3. Size the 7.62x39 to 6.8 and use a 6.8 or 6.5 seating die.
4. Shoot the "6.8x39" or "6.5x39" in your 6.8 or 6.5 AR using Frankenmags.
5. Pic up your fire formed twice fired 6.8 or 6.5 brass.
If you have an MGI upper then you just need a 7.62x39 bolt and barrels in 7.62x39 and 6.8 or 6.5.
With the MGI lower and an AK 47 magwell and AR magwell you can stick with cheap, reliable AK mags and then transition to 6.8 or 6.5 mags in the AR magwell.