My limited(non combat zone exp) GIs are a grab bag. HK mags start out great but once they start getting dropped on rocks/concrete/etc or pass the 1k round limit the feed lips start getting bent and have feeding issues. Their weight is a big contributing factor to fall damage also. Pmags work, until their spines crack then its 50/50 on it going on. Lancers, havent seen/heard of their spines cracking yet but again with the metal feed lips that can get bent if dropped on hard ground.
Havent tested my new M3s much yet(AWB scare, lack of ammo, etc) but I prefer them to my LAWMs, mostly cause the Lancer's polymer body is softer(ie I can squeeze them) than Pmags.
As for the Corps banning/restricting Pmags due to incompatiability with the M27, a little birdie told me that theres a revised version with a modified magwell(closer to AR spec) that will fit MOE Pmags(and obviously M3s).