I'm a big fan of the P-mags. They are true 30 round mags, in fact most will actually hold 31.
Load 30 and there's no need to download to 28 like some other magazines require to avoid malfunctions, or to simplify seating a mag on a closed bolt
Also like that you can stipple the surface of the p-mags to improve your grip with wet and muddy hands.
I have mostly the windowless version. They have been absolutely reliable and they are a little cheaper.
I bought a couple of the windowed ones just to give them a try, and they are just as reliable. The window which i didn't really think I needed actually comes in pretty handy when shooting a carbine match.
A quick glance lets me know for sure if a mag is fully loaded with 30 or has had a round or two used before starting a stage.
Also would be helpful if you have done some tac-reloads when it comes time for you to have to retrieve and sort through your partial mags.
Not really "needed" but it is quick and convenient.
I'm buying more of the windowed next time
Hellfire, Doom, Watch the hatred spin
Beyond the speed of sound---
Fire it up, Let the engines roll
It's time to burn it down