My opinions (and from what I've read on various boards):
USGI - the best (especially non-imports), but can be beat up/shot out - so check condtion. You will also pay more for them. They are also Curio & Relic (C&R) eligible.
Commercial post-WW2 carbines:
Universal - early ones were supposedly OK, but the later ones (with dual recoil springs and open area on the stamped-metal slide/cocking handle where the bolt lug rides) are total crap. Many USGI parts will not fit in later Universals.
Plainfield - the best of the commercial carbines.
Iver Johnson - pretty good, especially the early ones.
National Ordnance - crap.
Alpine - ???
IAI and Auto-Ordnance - OK, for the most part, but what you would pay would pretty much buy a decent GI carbine.
The USGI 15-round mags are the most reliable. There are a lot of crappy aftermarket 30-round mags out there, good only for range use. USGI 30-rounders are good, but more expensive than 15-rounders and perhaps not as reliable.
For plinking, 110-grain FMJ ammo. For defense, Remington JSP or Winchester HP 110-grain ammo.
Links:
www.battlerifles.com/viewforum.php?f=4www.fulton-armory.com/Carbinemags.htm