USGI is the way to go. IIRC, the 20 round USGI were no longer manufactured after something like 1971, so no worries whatsoever with whether they are preban or not.
Manufacturer of USGI stuff doesn't matter, except that you don't want 30 rounders by Cooper (less than 6 spotwelds). Some will pay more for Colt mags, but Colt didn't make them, so unless you're a collector, there's no point.