To be honest, I get whatever has the best price or value at the time. I haven't had any problems with the metal or polymer mags. The polymer mags are overpriced for what they are though. I mean seriously, eighteen cents worth of plastic for how much?
Honestly, they Pmags are almost the same price as GI Mags these days. They're about $12 and you can step on them, drive over them, and they won't bend or break and will still feed your gun if you needed them to.
If you only ever shoot from a bench at the range, then it wouldn't matter.