The upper is mil-spec from PSA and cost $280 plus shipping when on sale, the BCG is mil-spec and worth $140.
The NFA lower is polymer and has had the LPK changed out. These NFA lowers are about $150 ready to pin to a upper. Polymer lowers have a poor reputation for reliability. The new poly lowers from NFA seem to get much better feedback, lot of guys buying them.
Might be a good buy at $500 but not at $600.