I would go with Rock River because they have been out longer, have a good track record, and personally, I havn't heard anything about CMMG other than their 22 AR conversion kit. I'm not saying CMMG is bad, I have their 22 conversion kit in my AR and its great, I'm just saying Rock River 308 has a long proven track record and I have heard more about them. Rock River also has more styles of 308 to choose from. But in the end, its your $, your decision. Me personally, if I had the $, I would buy a Rock River and CMMG 308
But I'm not rich, just a common man who likes guns and buys what his $ allows him