Real USGI are gtg; I have preban Labelle, new D&H (and Bushmaster banded D&H) and recently picked up some NHMTG straight 20s (they supply Colt's straight 20s). Haven't used the NHMTG mags yet but the Lebelles and D&Hs have been 100%, they have the green followers. I think the Labelles ran $14, the D&H $10 and the NHMTG $13.
I have several C-products "USGI" mags. They are aftermarket pure and simple. They are not as nice on the welds but they work; so far 100% and they to have green followers. They were $7.
I have 1 Brownell's "USGI" mag; I don't know who makes it but it works. It was free.
I have several windowed P-mags I use in my MSAR E4 and Sig 556. The only difference I can tell is the P-mags are longer which makes them easier to seat with 30 rounds on a closed bolt. I use them in the 2 rifles above only because they look better, more at home, than the USGI ones. Of course, the USGIs work here too. They were $17 a pop.
I guess long story short it doesn't matter, lol. As long as it's a Magpul or USGI contractor they are good; just get what you want, that's at the price you want. It's probably easier to agree on which mags NOT to buy: Promag, eagle, triple K, USA, MSAR XM117and certain Thermolds.