I would just like to put in a heavy plug for what Jim Watson said. Unless you really want to deer hunt with your AR (6.x), hunt bigger stuff (.50 Beowulf), or...I dunno, can only shoot pistol ammo at your range (9mm) I can't think of any reason to buy anything but a .223/5.56 rifle.
This round or that round may be better but for plinking, target shooting, varmint hunting, and home defense 5.56 will do just fine. And it's both cheaper and more available than most other options. Pistol ammo, 9mm and .22LR, is cheaper. But of the center fire rifle calibers the only thing that is even close is 7.62x39. And, due to the nature of ARs, you can always buy an upper for a more exotic caliber later.
Other than that, you have lots of choices. I would have no problem owning Armalite, Bushmaster (which I do own), Colt, Rock River Arms, or DPMS. The only reservation I would have is that, from what I've heard, a Colt may not be compatible with other brand uppers, due to a difference in pin size. Could someone chime in on that?
Here is my advice on options. I'm a minimalist, so take it with a grain of salt. Don't spend money on lots of options, unless you have an articulated need for it now. The AR is extremely modular. You can always add on different stocks, grips, rails, and all the accessories that go on rails later. It's easy, most can be done with a screwdriver and five minutes. Buy a stock AR, shoot it, find out what you need and like, then start adding on.
The sole exception to my advice would be getting a rifle with a removable carry handle and a rail for mounting an optic sight. That is the one thing that is hard to work around. It is very difficult to mount an optic sight on a rifle with a fixed carry handle. There are adapters and such, but most of those will put the sight very high up. You'll probably find yourself wishing for a removable carry handle and it's cheaper to pay the extra $100 now than to spend several hundred on a new upper or even a new rifle down the road.
