At the top of this forum are some great FAQs. The ranking thread by Troy is apropos.
20s are great..........30s are great. Use whichever you like, but aluminum USGI spec are considered both definitive and the minimum standard by most. Sounds like you are very well set.
FWIW, I use 20s and 30s. And some are old "pre-ban" and some are newer.
I used 30s that had the now passe black followers long before I ever heard of this site and green followers. I should be ashamed of myself for forcing all those rounds to their death by way of black plastic. Funny, but they never complained............or tipped.
As for 20s most seem to prefer the old mags with alloy followers..........but then, the new ones with plastic followers work every bit as well. I even have some curved 20s inbound. We'll see.
Sam