If you're a left-handed shooter, I'd run, not walk to ArmsEast and pick up a STAG! Two of my buddies are lefties and they vow that'll they'll never go back to shooting right handed rifles.
As for your choice of manufacturers. If you can bear shooting a right handed rifle, there are some good deals on used Colt sporters. Colt has the "brand name" recognition and the Kool-Aid Drinkers on the go vow and declare that if it isn't Colt its junk! I own several Colts and they're fine rifles, but they aren't a quantum leap beyond my LMT Defender or STAG. In fact, the LMT seems to be the better made firearm, however, it doesn't have the same resale value that a similar Colt LE6921 would have.
Colt seems to have become progressively anti-civilian so the forces of supply and demand have driven the prices of Colt M4 clones far higher than they're actually worth. LMT and STAG are making equally as good firearms, especially for sporting purposes, even though they're made to meet/exceed mil-spec requirements.
Rock River Arms are making good rifles these days, as are Armalite, but I've found that LMT & STAG seem to fall within a higher tier of quality. Bushmaster, DPMS, etc...seem to be hit or miss. Some are exceptionally well built rifles, others are simply horrible, although Bushmaster seems to have solved some of their biggest QC problems that have been plaguing them for the past couple of years.
Before making your final decision, use the search function for the Black Rifle forum; also, why not check out AR15.com? There is mountains of feedback, reviews, pictures, etc...on just about every AR variant ever made. Should help you a little in making your decision.