FWIW, the SOF folks I was working with in Afghanistan seemed to be exclusively using PMAGS.
I own PMAGS, CProducts, and some surplus metal ones. The only issue brand wise was one of my surplus ones causing some feeding issues.
One word of advice repeated to me and which I have experienced first hand, rotate your loaded mags. I had a couple issued mags I left loaded for 6 months and lets say the springs were a little loose when I stripped them empty for ammo turn in.
As for 20 vs 30, like a few have said, personal preference. 20s are a little easier to bench shoot with since they don't protrude so far. I don't know if this is applicable to your situation, but if you wish to hunt at all with your rig, and state laws mandate no more than 5 rounds, you'll need to add a few of those to the mix. I own CProducts 5 rounders in 6.8 and 5.56 and have experienced no issues. They also work great off the bench, as they protrude the least.