Not to make it hotter in here but using the Lexus, BMW, etc comparison to a Porsche is really not the same as comparing a Colt to the other AR-15's. A Porsche is more sporting, and have a slightly different purpose. The AR-15s are basically identical, and all meant for the same purpose. The quality of construction is probably the biggest difference between the brands of ARs. Some ARs come with features not found in others, but all use interchangeable parts....upgradeable using parts from other ARs.
Here's the thread I mentioned earlier....anyone looking to buy an AR should read this:
http://forums.officer.com/forums/showthread.php?t=81462