I just saw this and don't have the ballistics in front of me, but dig into the 6.5 Grendel. It is an AWESOME AR platform and accurate as you will probably ever be. I absolutely LOVE this round and can't say enough good things about it.
Best of luck and safe shooting!
That, and you can load a DT 200gr .308 with 1732ft/lbs at 500 yardsI would go for either 6.5 Grendel or the .308. The 6.5 has better ballistics, but the .308 is more common. Plus the .308 is a .308.