For 20 vs 30 - no curve to deal with, and for metal vs plastic - none of the extra reinforcing ridges, same size top to bottom, etc. Lets them be packed tighter with less wasted space from voids between them.
P mags. Less rattle and they always work. I don't run anything exotic, so I use Gen 2. I do have GI mags as well. It kind of reminds me of the Glock vs. 1911 debate.
20 round GI mags for me. if your shooting from a bench,which is what I do most of the time,there is not a clearance issue. as Sanford said, they stack better. fwiw, I have pmags also.