I'd buy more AR magazines. Try to keep things simple and interchangeable. That makes upkeep easier.
I'm coming to that conclusion. I stopped by an LGS today and they had a full wall of available ARs by different mfgs, which was nice to see. No 30rd 5.56 mags, however :-(
Assuming using an AR for practice and maybe some 3 gun competitions, etc down the road, I'm curious how many of each type of mag one would need: aluminum USGI, 30rd Pmag & 20rd Pmag?