IMHO the quality metal Mags are the best choice for a long lasting Mag. if your going to be depending on it to work when needed.
I read the Magpul Mag. lips will weaken when kept fully loaded. This and the fact Magpul has had the their recalls and they themself have posted they are having quality problems in some production runs.
They might be a good cheap Mag. for range and plinking but as a real defence hardware item I say NO WAY.
UMMM???? Magpul PMAG's come with a dust cover that is also a relives pressure on that feed lip. So that stuff doesn't happen when stored for long periods.. I think that the PMAG's beat metal mags any day! Metal mags have the feed lip problem too. Ask around. Also, those problems with the mags not ejecting was corrected. They will tell you that too.