Out to 300 yards not much difference at all. I get 2425 fps using hornady 120 sst and 2775 fps with the federal xm6.8 90 grain gold dot out of my 6.8 spc ii 12.5" ARP barrel in my sbr. Both kill nicely! I was going to build me a 18" upper also, but after seeing the velocity of the 18" barrels getting a whopping 100-125 fps more then my 12.5" i decided against it. Very efficient down to 12" i must say.

I have tried to talk myself into building a 6.5 Grendel for a couple of years and just haven't been able to convince myself I need one. I built two 6.8's, one for deer hunting and one set up for night hunting hogs and coyotes. I have more ammo than I will live long enough to shoot, let alone brass and bullets for reloading. Either one will fill the need and I don't get into the huge argument that arises when fans of each caliber go nuts about which is better. Shoot what you want and don't worry about how someone else feels about it.

I went looking for something better than my 223 AR and SKS and found the 6.8.
I've used 6.8's for years exclusively for pig hunting and average 70 pigs a year with it.
Once I found AR Performance and their excellent barrel/bolt/chamber, I never looked back.

98 percent of my shots are between 75 and 130 yards and the 6.8 serves this purpose quite well for me.
I would like a 6.5 to play with but am heavily invested in 6.8 so it will have to wait.

I chose the Grendel over the SPC for the potential to use bullets with higher SD. It just so happens that SD is a part of the BC equation, but external ballistics is low on my list. Terminal ballistics is high on my list.