6.5 has better ballistics, but IIRC it's a "proprietary" cartridge, and reloading dies and ammo are not cheap.
6.8 is closer to a "mass produced standard"
I'm leaning towards 6.5 as a balls to the walls long range "hunting rifle"..
6.8 would make a better CQB with some oomph approaching 308 gun
MB nails this one on all counts.
I own 3 6.8 SPC AR's and in my mind the 6.8 makes the AR everything it should have been the first time. I'm biased. My favorite AR of all time sports a 16" light weight barrel chambered in 6.8 SPC.
There is reams of info on the interwebs going back several years, debating 6.5 vs 6.8
IMHO from a pure performance standpoint if you are looking for a true long range round (500m and out) go with the 6.5. If you want steller performance from 300-400m and in or CQB the 6.8 SPC wins. If you are considering a SBR the 6.8 SPC is your choice. A 12" SBR in 6.8 SPC may be the ultimate CQB AR.
I picked 6.8 over 6.5 because I don't shoot anything beyond 300m and I love it's CQB potential. It's also a great round for deer, hogs etc. (As is the 6.5) I've seen first hand what it will do to a 200lb feral hog and it's impressive.
I also don't like the fact that Alexander arms holds the rights to EVERYTHING 6.5. If you want to play with it you either have to buy or license it from Bill Alexander. This has really slowed developement of the 6.5 Grendel. In comparison every major AR manufacturer except Colt offers weapons in 6.8. The 6.5? Not so much.
Plus and most importantly the round just LOOKS right. I know you guys get that. The 6.8 just looks like a combat round.