I'm floating back-and-forth between 6.8SPC and 6.5 Grendel for my next AR. I have a standard 5.56/.223 AR (16") and a .308 bolt gun (24"), so a midrange round in a midrange gun (18-20") is very appealing to me.
From what I gather, 6.8SPC gives a great middle ground between the two NATO rounds (5.56 and 7.62), but the 6.5 Grendel seems like it fills the same void, while grossly intruding into the 7.62's turf.
On deer-sized game at medium ranges, either would be a good choice. The fact that Wolf is loading some cheap (brass) ammo for the 6.5 Grendel may just put me over the edge that way.
I've gotten into reloading, so I'm toying with the idea of a 6.8 upper. I've got a RRA 5.56mm upper, and its accurate, but with all these other calibers with superior terminal balistics available, I wouldnt want to use it for anything other than punching paper, especially since I'm gonna try to go deer hunting sooner or later.