Buy once, cry once. Read up on the subject before spending your money. Here is a good start,
http://www.m4carbine.net/showthread.php?t=6642
Click on "Link of Google Doc Chart"
Well,
somebody had to say it.
After looking at that chart it seems the Colt is the standard the others are measured by.
You are correct, and have saved yourself years of hassle and at least a few hundred bucks selling off Tier 3 and Tier 2 rifles working your way up. However, that can be a worthwhile and valuable journey in itself. I'm glad I did.
The BCM looks close in quality and a little better in price with a lifetime warranty. Will the BCM hold it's value as well as the Colt ? That's another question.
I have been very pleased with the performance and value of my BCM upper/LMT lower, running it hard in high-round-count classes and non-square range drills. IMHO, Colt will hold its value better. FWIW, the 6920's that I've fired in class have shown excellent F&F and reliability.
Flexxx (OP) and mknopfler, although I am a big proponent of ARs made from the
correct materials properly assembled (a la The dreaded Chart), your end use will ultimately determine which rifle is "best":
Fighting or class rifle? Stick with The Chart, and read m4carbine first.
Shooting coyotes or groundhogs at 400m? Reliability not so critical. Get a tight .223 chamber, aftermarket trigger, and a long barrel.
an AR just to "have" an AR (2x annual gravel pit shooting)? Anything that catches your fancy will do just fine. Read ARFCOM.
Again IMHO, some of the best values in the AR world include:
- S&W M&Ps when they can be found for ~$600. Google around, even brick and mortar stores on Black Friday.
- a surplusammo.com complete lower for $165 and a BCM upper of your choice w/ bolt carrier group ("BCG") and charging handle ("CH") for $534. Add basic used handguards for $15.
- Build your own lower with a quality lower parts kit ("LPK") and put a good upper on it (BCM, Daniel Defense, LMT, Colt). Installing an LPK in a stripped lower is not that hard (in retrospect), but was a LOT harder before an armorer showed me how!

To provide a knife example, a knife made from AUS-4 will serve many people's cutting needs, but a dedicated hobbyist or dedicated professional user will want something more. That being said, many police miraculously survive each shift with Bushmasters and Maxams.