First you need to decide what you want to do with it. Colt's and Armalites are good weapons, but they are not as accurate as some other ARs on the market.
I perfer Rock River ARs. I have a Rock River National Match rifle that will place a dime size five shot group at 100 yards. Research what accuracy the factories claim before you make up your mind. If you plan to use the rifle for hunting and you want something that will work clean or dirty, the a Colt or Armalite is the ticket. A combat (hunting rifle) is not built to the same exacting tolerances that the S&W MP15, RRA, and some of the other ARs are built to. My RRA AR is very accurate, but I wouldn't want to take it to Iraq or Afghanistan.
+ 1 for the S &W M&P 15 MAGPUL version, I've been very happy with mine, also have the CMMG .22 conversion kit for plinking. Rifle is nice and tight and goes bang in a very reliable fashion, quality is good and so is accuracy........and at a price slightly lower than a Colt. There was a great spreadsheet on AR.com I think that had comparisons of most ARs with pros and cons and also nice to haves and must haves like chromed barrel, 5.56 chamber, M4 feed ramps etc etc
Personally, I prefer the MILSPEC 1:7 twist barrels. Nice to be able to shoot the heavier stuff if you want to, but I personally prefer the LMT rifles. I've had a few S&W M&P15's and they were great, but not nearly as nice as the LMT I bought. LMT is the one of the only companies that build the 10.5" rifles for SOCOM. As a few have said, snag a lower, and buy the upper of choice.
M4 CHART this chart is helpful. Its how I decided to save for a Daniel's Defense instead of build my own. It does compare most/all major brands.
Sorry I just noticed this......
1 March 2011 Edit: .
For a variety of reasons, the Chart in all forms has been removed from this document. What remains, therefore, is the Explanation of Features. Give a man a fish, you feed him for a day. Teach a man to fish, you feed him for a lifetime. With that in mind, the fish has been removed. What remains in the interim is the fishing rod, reel, line, bait, and a map to all the best fishing holes. Educate yourself, and use them. .
The Chart is not "dead", but it is offline until a new version can be completed. No, I don't have an ETA on that.