Griffin Armament Alpha or Sig Srd762.
The Silencer Co Omega got my vote. Light, quiet, good mount.
But minimum barrel length for Omega on the 6.8 is 16". If you think you will go SBR, then I would suggest the YHM Ultra Light Tactical.
Silencer Co has always responded to my emails quickly. On the 6.8 and 16" minimum, they have responded that the capacity of the 6.8 is what kicks it up to a 16" minimum barrel length. Doesn't seem to make a lot of sense to me, but they're the guys who designed, manufacture and test the Omega, so who am I to argue.
Btw, the muzzle brake really does work to reduce muzzle rise with a 16" 6.8. I don't notice as much improvement with the 5.56, but it doesn't have as much to begin with.
JPK
I went with the TBAC Ultra 7. At 11.5 ounces, it is a lightweight sucker, and is rated for a 12.5" 6.8. In process of building a 12.5" 308 and ordered a YHM Phantom Ti DT. 15 ounces and rated for 12.5" 308.