go to AR15.com and check out their mag facts section, you will find a wealth of information on all types of mags.
For the most part USGI is the way to go, you can add green followers for $1 each if the black follower jams in that mag, out of about 40 USGI mags I have and use only about 10 have green followers and they all work great.
Do not pay any extra for a Colt marked mag as Colt didnt' make them anyway, they are just marked that way, I have used good USGI mags from Parsons, Okay, FN and others, just make sure they have enough welds in the back.
As for 20 rounders, I use the old USGI with metal followers to shoot highpower and CMP and they work great.
The plastic mags work in some rifles and not in others, I tend to use them in three gun where i'm dropping them on a hard surface when I dont want to screw up my USGI mags, AR15 mags were never designed to be that tough, they are light.
The steel mags from England are tough and last forever, they work just like USGI just a bit heaver and well worth the $18-22 they are selling for, Dillon is selling them for $20 with green followers.
Welcome to the world of ARs you will never go back to the mini14 after finding out how the AR kicks its butt.