Although the 6.5 Grendel is capable of 1000+ yard shots, at that distance, I'd go with a good Remington 700 Police/sniper in 300 also. For a couple of reasons really. The biggest being weight and manueverability, the second biggest is downrange velocity in a closed bolt system will be much better than a semi action that uses some of the gasses to cycle the next round for you.
If you just have to have it in an AR platform, the DPMS would be my choice because the Knight's SR25 308 is just way outta my price league all at once for about the same thing minus the military contract numbers for around 1/3rd the price in the DPMS setup.
My 2 cents