AR enthusiasts (myself included) tend to group their rifles into two broad categories. One style is to maximize for performance in terms of accuracy. These rifles benefit from match barrels which float free in the front stock and good quality scopes. Another style is to approach for more of a practical shooting weapon. Short barrels, muzzle brakes and red dot optics without magnification become much more interesting. Both are amazingly fun to shoot.
Some people will use interchangeable uppers (barrel, front stock, upper reciever, bolt and carrier) on the same lower (lower receiver, grip, trigger mechanism, buttstock etc.) in order to get more options out of the same "gun" (legally, it is the lower receiver that is the "gun", everthing else can be purchased through the mail without having to fill out any forms). I am not personally interested in doing this because the trigger mechanisms I want in the different kinds of applications are not the same.
When you look at the quality of construction on AR's there is a lot of variability. One thing to look at is the metal to metal fit-up of the upper and lower. Poor quality fits will have a lot of movement. Good quality has little or none. Good quality receivers are forged, machined and then heat treated.
Barrel quality is another issue. The military uses chrome lined steel barrels which withstand some serious punishment. Accuracy afficianados have recommended to me that the best barrels for them are actually stainless and cryo treated. Various options exist as to how to treat the end of the barrel. I personally like having a flash hider on there, not because I worry about my night vision, but because I like not having to worry about dinging up the crown of the barrel and destroying its accuracy. Some people have proposed that the Vortec flash hider actually increases accuracy by directing expelled gas away and not destabilizing the bullet in front of the barrel. Maybe this is true, but I can't add an opinion. I can confirm that there are some muzzle brakes on the market that do an amazing job at recoil elimination, but I don't know them by name. The recoil on a 5.56 is pretty moderate, and the main reason that these muzzle brakes are interesting is to put out a series of rapid accurate shots to hit a plate rack. Just don't stand right next to someone using one. It's really annoying.
I also have an AK, and I have to agree that the ammo is cheaper. Also, I just can't care about the thing as much, even though it's a nice Bulgarian model. It just isn't made with any love. As a result, I can't seem to feel anything for it. I wouldn't want to use it for my pseudo practical shooting activities because I can't come on and off the safety without fumbling for several seconds. It's surprisingly accurate at ranges appropriate for iron sights.
There is a book I have somewhere (I'm at work so I can't check my home shelves) which was a compendium of information about the AR-15. It was very informative for me when I was first thinking about these rifles. I learned about the process of manufacture and some of the history behind them. The author also did some side by side comparisons of a number of models and I also found this to be informative. At the time that I read it, there was a sequel expected and it's probably out now. I recommend doing some reading. It will make you a more informed consumer.
Personally. I paid quite a bit more than you've proposed for my first AR. I was looking to put together something that I wouldn't want to upgrade a bunch of times. I have no regrets. However, it led to the need for buying more weapons... what a shock!
