I am putting together three uppers, and I want them to be identical. I am going to put a flip up rear sight on each one. I was going to put three plastic Magpuls on each one, but now I see that PSA is selling an aluminum sight for $40.  Can anyone recommend one sight over the other?  Does yours stay put and is it easy to adjust and use?

Magpul has been around forever and is a safe bet. I've yet to have one move. They're a little ugly IMO but acceptable by majority standards.

I've had my eye on the aluminum aero precision flips. Not very pricey and low profile.

Magpul known quantity for sure here.  Chances are they are more durable too.

I bought a few of the Palmetto's during their last sale. I'm pleased with mine, it's well made, all metal and very solid. The thinnest piece of the sight are the aluminum "ears" on both sides of the aperture, and even those are still overbuilt and almost 1/4" thick.  The sight is not spring loaded and does not lock open or closed. Press it up or down with your finger and it clicks solidly into place. I know the magpul gets high points for being tough, but I just like an all metal sight.

The only possible issue is that folded sight is not the lowest one around, but is still clears all my scopes.

I checked mine, and it's .610 inch above the top deck of the picatinny rail when folded, and clears my largest scope ocular with 1/8" to spare. Specs for the polymer magpul is .510 inch, so the magpul is a bit lower.

I like the sight, it's solid and a great buy on sale.

The PSA sights are an awfully lot like the folding sights sig includes on the M400 rifles for what it's worth.

Sig:

http://www.sigsauerguns.com/sig-sauer-hodge-rear-flip-up-sight-sight-flip-rear-1.html

PSA:
http://palmettostatearmory.com/psa-aluminum-flip-up-rear-sight-38183.html

Good catch. I have my palmetto right in front of me, it looks completely identical. As I posted earlier, I'm pleased with it.

If you're going with aluminum sight you might as well use the cheap sights on eBay that are like $22 shipped.

I'd go with the Magpul's.

I read a review somewhere (I'm sorry I no longer have a link) where a guy dropped a few AR's on their BUIS and then tested them to see which held their zero. The plastic magpul's held up the best and basically never lost zero but all the rest either did or even broke (even the MBUS "pro" models). MBUS is the way to go.

The MBUS Pro was NOT in that test.  The reason the poly MBUS did so well was that it does not lock rigidly upright when deployed.  It is spring loaded.  That permits it to retract rather than absorb the impact.  The MBUS Pro has a detent when deployed that also lets retract when struck.  So do KAC micros.  MBUS Pro did not even exist in 2012 when that otherwise excellent test was done.  The poly MBUS is a tough, effective budget backup.

https://kitup.military.com/2012/07/back-up-iron-sight-drop-test.html