Best parts kit? Probably Barrett followed by Bushmaster, but they are the most expensive options as well. Cheapest option is probably www.model1sales.com I have a .223 Upper assembly from them and it has worked well so far but many of the members at ARF.com say they are junk and only Colt is good. DPMS also makes an upper assembly in 6.8SPC. I think if I delve into 6.8 SPC I will get the DPMS. www.jtdistributing.com sells lower receiver small parts kits for like $60 and they are high quality. Better than Model 1. I heard from someone that JT's lower parts kit is a Colt equivalent.
I feel that if I get a 6.8SPC AR I will use it for 0 to 300 yards and more of an open sight or low power optics rifle. I plan on getting a 6.5 Grendel for doing some more serious accuracy shooting and going out to the longer distances. 6.5 Grendel is better all around in my opinion but it's good to have options.