I have a Bushmaster DCM-XR A2 with a Hogue grip, a MagPul trigger guard and a free floated barrel. I have big hands, and I just could not live without the grip, and the MagPul trigger guard (curved or straight) is awesome. It eliminates the gap at the top of the grip that causes blisters, and it beats the hell out of that crappy plastic one Bushmaster uses. The free floated barrel gives me around 3/4 MOA out of the box. Wonderful rifle for anything 100-600 yards. I also have experience with a friend's RRA set up in the same format. Same accuracy, but some say more reliable. I haven't had problems with either.
However, I get the feeling you're not looking for this kind of rifle, your're looking for a carbine. If I were to get a carbine, it would NO DOUBT be a Smith & Wesson M&P-15T. I'd of course put an Ergo Grip or a MagPul MIAD grip on (do it on the kitchen counter, costs $30) and a MagPul trigger guard, curved and non folding (if you have the right punch you can do it at the counter, $20). I'd also probably get an EoTech 512 and that forward vertical grip that converts into a bipod at the touch of a button, but that's not really relevant. I love the M&P-15T for its reliability and longevity (it can go 20,000 without ONE part being changed or repaired, including springs, without a loss of accuracy or so much as one hiccup, believe me I've seen it firsthand), for its lifetime warranty and for the fact that it comes with a $400 complete set of Knight folding backup sights compatible with EoTech, Trijicon and AimPoint sights, free. If I were to make a major change to the 15T, it would be the addition of a folding MagPul stock that eliminates the nasty little wiggle in the standard folding carbine stock. (Doesn't affect function, just makes you go "Eeeeeeewww!" when you seat it to your shoulder.) The M&P-15T goes for around $1,300, but trust me; if you get a $1,000 gun you're going to spend another $300 turning it into the 15T anyway, it's just inevitable. You may as well buy it as a rifle and get it over with. Then you'll only have to spend $50 on mods. I've put about 1,000 rounds through my buddy's 15T, not a single hiccup. It was an absolute RIOT to shoot, too. I don't see how you can go wrong.
As for the chart above: Sure, there are advantages to having your rifle be mil-spec. You could take a Colt, tie it to your bootstraps and wade through sand, muck and show for ten miles, then shoot it on full auto until you could cook steak on the barrel. But I've seen Colt M4s that couldn't hit the inside of a barn.... from the inside. You may not be able to do that with a Bushmaster, RRA or a S&W, but you'll be able to put rounds on target at 600m. I'm not saying Colts aren't accurate (mainly because Hunter would gut me like a fish), I'm just saying that you could do better in that area, and that if you don't need the main advantage Colt has to offer (reliability in adverse conditions) you may want to get your AR from a company that specializes in something else, such as accuracy.
I'm no expert, so all you AR people feel free to tear my post to shreds, I don't mind. :flame: Hope I helped!