I use both, because I have both, but put me down for P Mags as well, I like the windowed ones, but honestly never pay attention to it so the solid ones work just as well.
P mags are good. I have both though, and use both. Mostly use metal for coyote and varmint/vermin control. I have several 20rd metal mags, they just seem to handle better in a stand or laying prone on the back deck.