I just ordered a 8.5" 300BO upper from Radical Firearms a few days ago. It was on sale for $199 and I have read nothing but good things about them. Plus they are based out of Texas and Primary Arms says they are G2G. You cannot go wrong with PSA or Aero. I have a SiCo Omega now and two other cans in jail that will work for this build.
I have built and bought many uppers over the last few years. Unless you really are going to build it from the ground up (certain rail, certain bbl, certain gas block, etc) I do not see any advantage to building it. The cost savings and quality are a huge factor in buying a complete upper from a well known manufacture.