6.8 for me.
The Grendel, while a big improvement, was merely designed to be better than the 5.56.
The 6.8, on the other hand, was developed by those with boots in the sand, so to speak, to provide the proper balance between distance, accuracy, and performance necessary for combat.
Bidah (I think) was right about the Grendel having more load variety, but that's not how combat loads work. You don't get to stop and think about which load to use based on the situation. You have A load (produced in the billions) by lowest bidder.
For reloaders, the 6.5 is top dog. For ground pounders, though, the 6.8 appears best.
Of course, that's just my free opinion and I always promise to deliver what you pay for.