Don't waste money on the lower; anything that says "MILSPEC" is fine, it's those hand-tuned trigger groups that will cost you, and if you know your way around an Arkansas oilstone and
buffing wheel, not even that. the upper is where you want to put your money! get a nickel-boron-coated bolt and carrier and a quality stainless or nitride barrel. Chrome is for lazy guys who never clean their guns and don't mind a MOA here and there.