One works in armalite pattern, one works in DPMS patterns. I forget which is which, type A is Armalite? Anyway, PMAGs are DPMS pattern (DPMS pattern mags are also called SR-25)
Armalite Models come in 2 varieties, the AR10A models take PMags, the AR10B models take the Armalite proprietary magazines. The A model came out after the B model, so check before you buy.
Q