MacCool

IMHO, the upper is a different kettle of fish than the lower. The upper is where all the action is and to have reliable function the quality of the individual components and how they interrelate is more critical. The concept of "tolerance stacking" really applies here. An upper receiver that's only a couple of thousandths off might not be a big deal, but if your BCG is also a couple of thousands off, and the bolt is a little off, and your chamber is minimally out of spec, and your gas port is slightly off, and your feed ramps are a little mis-cut, and the barrel extension indexing pin is off, and your front sight is canted, now you have a potential reliability problem. I've seen all of these things in various combinations and over the years I've found that sticking to the known higher-quality mfgrs minimizes those troublesome issues.

You'll often see opinions posted on places like Arfcom that "with Colt, or Noveske, you're just paying extra for the name". I've found that what you're actually paying for is a higher level of quality control. They throw away a higher percentage of components that don't meet their more rigid specs, so the chances of tolerance stacking are minimized.

While I wouldn't be very concerned about buying a lower from PSA, I don't think they'd be my first choice for an upper. I would have zero concerns about buying a complete upper from BCM, or Colt, or Noveske, or DD, or about building an upper from components from those mfgrs (which is what I've been doing lately with the SBR's I've been building, or rifles I help friends build).