Magpul will definitely stand up to abuse better, and I've had zero feeding issues with them. Lancer brand polymer mags also seem to be getting a good rep, and since they have metal lips I don't believe you need to keep them capped like you do the Magpuls if stored loaded.
I can't speak to the reliability of USGI mags since all the ones I own have CS springs and Magpul followers in 'em which makes them work just as good. You may have slightly more to worry about when you drop them during training, since the feed lips are more fragile than the poly mags. Personally, I use a Magpul in the rifle and have USGI mags in the go bag, since I don't want to have to remove the storage covers in a hurry. If I was required to use USGI mags, I'd swap the stock springs and followers for what I said above and be perfectly happy.
On a related topic, when I get my uniform allowance in April I'm gonna purchase a 60-round Surefire AR mag to keep in the gun. That ought to buy me a few minutes time to resolve any problems I come across.