I was bitten by the black rifle bug rather badly so I bought, traded and sold a large number of them. That experience may be of some help.
I prefer A2 configuration and shoot only iron sights. I've fired 7 years worth of 200 and 600 yard matches with an AR-15 and even shot a large number of 600 yard matches with A2's using Winchester White Box (WWB) 55 grain bullets in a 1:9 twist barrel. That is some cheap ammo but it performed very well at 600 and great at 200 from 1:9 twist barrels!
My best results at 600 prone matches came from a 1:8 twist using 69 grain Sierra Match Kings. Barrel twist rate is crucial on an AR and your barrel choice will determine what ammo you can use.
The 1:9 twist barrel is the best overall and you can use the cheapest ammo (55 grain) in it. I did poorly in a 600 yard match using a 1:8 twist and the 55 grain bullets. They may be OK at 200 but at 600, that barrel/bullet combination won't cut it.
SO.....my suggestion? AR-15, 20" bull barrel A2 configuration with 1:9 twist. Get the solid butt stock if you intend to shoot at distance. People swear by certain brand names. I've had about all of them and just don't have a preference. All are made on CNC machines, now and quality is very high.
The AR that I shot the most matches with is an Olympic that I picked up at a gun show. The rifle was a bit homely and had a fake suppressor on the barrel. I planned to use it just for dry firing................until I fired it. That rifle shot great and I grew to like it so much that I preferred it over the others. It is a veteran of countless 200 yard matches!
From my experience, the only truly unique AR's are built on "Plum Crazy" plastic lowers and "Sun Devil" billet lowers. Plum Crazies may not be appropriate for distance shooting but I have never owned one. My "Sun Devil" billet lower is a winner. It is VERY different from the pack and is very high quality.
I have never owned a lower housing from any source that I consider substandard or junk.
Have fun!
Flash