PMags - it took me a full 15 minutes or so to bust my first one at the last course I took.
They're NOT that tough at all - just run up on some cover, and misjudge your breaking distance ;D I've kept the ones I've got as square-range mags, but not for serious mags.
Personally, I much prefer the HK mags. If you don't mind paying 37FRN per, that's the way to go.
For the same dough as the PMag, you can get a CProducts stainless steel bodied mag. Those are 20 thou thick, and good.
I've had great success with the SAR80 mags. They're 30 thou thick (same as the HK 416 and M14 mags) and solid steel. Only downside is that they're parkerized steel (rust possibilities if not maintained) and the follower isn't upgradable. Still, it's a US GI follower, and it gave me ABSOLUTELY NO problems in the monsoon or desert. Both with the XCR. Might need a touch of break-in (or fitting), but they're good 'nuff that I've got about 73 of 'em.
Did I mention that the SAR mags are cheap too? Same price as a cheap US GI mag - and just like one - but in steel.
An aluminum mag? Well........ I don't care for 'em for the same reason I don't much care for PMags.