Most mag's work great depending on the type of spring and follower that they contain. Replacing these two items will make most bad mag's into good ones, regardless if they are made of metal or Plastic , unless the lips on the mag's seriously deformed .
Yep. My take on AR mags is this; P Mags are plastic (duh!), this means they won't bend. GI mags are aluminum, that means they won't crack. It's kinda like comparing aluminum arrows to carbon arrows, pick your poison. You have to provide reasonable care to either one of them.
I've seen a lot of abused Pmags in use by GI's. A LOT of these had feed lips that looked like they had spread a bit. With vigorous magazine changes, sometimes these mags would spit a round or two out of the magazine as it seated, of course, this will jam the gun up, especially if it is a slide lock reload. I think this happens to GI's 'cause they all LOVE to do full auto mag dumps, one after another, thereby rendering said Pmag thoroughly well done.
If you are gonna leave Pmags loaded, you probably are well advised to use that little cover that takes the tension off of the feed lips.
A GI mag that is within specs for OAL width etc (not all are, but that is a story within itself) has a magpull follower (I like the yellow ones) and a chrome silicon mag spring, is as good as it gets. Download 'em to 28rd so you can do a tac-reload more easily.