I have no interest in SBRs but if I did did make one I would agree with ED308 and make it a 12" 6.8.
The ongoing debate between the 6.5Gren and the 6.8 SPC is something I have a lot of real-world experience with as a gunsmith and as a hunter.
In a nutshell I call it this way:
"6.8 for short barrels. 6.5 for long barrels."
Having killed 33 head of game myself with 6.8 and seen killed about 30 others. I have reports from about 60 more kills on top of those from friends and customers that use my guns. I have a good idea what to expect from 10 yards to 435 yards.
I have killed 6 with the 6.5 Gren and seen 8 others killed. Ranges for the 6.5G kills were between 60 yards and 385 yards. I also own a chronograph and so what I know is knowledge, not theory.
For performance in the game-field it's best to think of the 6.5 as a rifle shell and the 6.8 as a carbine shell. The 6.8 SPC does gain more speed between 16" and 20" than I thought it would, but nothing I can do with a 6.8 will beat the 6.5 in 20 to 24 inch barrels, and the .277 bullets don't hold their velocity as well. (they hold it well enough to make it an honest 1/4 mile gun, and that's all I'd want from it anyway)
With 125 Gr .264 Nosler Partitions, the 6.5 Gren is an honest 600 yard hunting gun. It's accurate even farther, but not all that deadly out past 600 as the bullets that can expand at 200 yards don't open much (or at all) at 700.
So the Gren is a better target rifle (by far) but the 6.8 kills a bit better at close range out to about 250. At 300 yards I can't see any difference on game as far as the size of the wounds and how fast they drop.
The other side of the coin is this;
loading for 16" and shorter barrels, (which makes the Gern fans mad, but it's a fact) the 6.8s I have made actually out run the 6.5 Grens I have made in 16" and shorter barrels, and sometime out run the Gren by as much as 125 FPS.
You can shoot the 125 Gr Nosler Part in the 6.5 and that makes it the best killing combination I have seen in that bore size. Most of the .264 bullets I have tried for long range flight and for accuracy were so-so on game. Not the Nosler 125 partition. It's a perfect bullet for the Gren. at nearly 400 yards it worked perfectly and at 30 yards it worked perfectly. I have been told by a friend it's fine at 550 too, and I believe him. (I was not there when he killed that big deer, but I did the butchering for him and the wound through the ribs and meat behind the foreleg was perfect.
The 6.8 SPC works well with a handful of bullets on game. From 85 grain to 130 grain. In fact, other than the 90 grain Speer TNT, all the most accurate .277 bullets I have tried have been hunting bullets. The Sierra Match king and the Matrix bullets doing well, but not as well as the Barnes, Sierra, Nosler and Hornady bullets.
The 90 Gr TNT was one of the most accurate bullet I have shot, going under 1/2 MOA in several guns, but are designed to blow up, and even at the lower velocities of the 6.8 compared to the 270, they still blow up. Can't blame a bullet for working as advertised however.
Anyway coming back to the point of the OP, I agree with Ed308. If you want an SBR on an AR, the 6.8 SPC is probably the best choice you could make.