I just got my first AR, an Armalite M15A4.  Do you recoommend 20 or 30 rd mags?  Why?  What brand is best?
Thanks

elpaso,  

First welcome to the black rifle club.  Get a few of each as long as their USGI.  You want reliablity above all else.  20s are great for prone and DCM shoots.  30s take a long time to unloaded but are fun.  If your in the El Paso area you should have no trouble finding USGI mags.  If you decide to hunt with it, personally I use a 20.  (last time I hunted Texas it was legal to use high cap mags).  There's alot of coyotes out on Ft Bliss North and South.

De Oppresso Liber

Both have merits, so I suggest you score a few of each. 30s = fewer reloads 20s = more freedom shooting prone (esp. for competitions). Assuming you are shooting semi automatic, either SHOULD be adequate for your needs.

I am partial to my brand(s) of USGIs but the concensus is that ALL are comparable (particularly with a new spring and a green follower) with the exception of Coopers. These 30 rounders may have 3 welds instead of six and are of substandard quality in terms of fit. When buying a USGI make sure that the feed lips aren't cracked and that the floorplate tabs aren't broken. Check out Magazines FAQ and good luck to you.