IMHO, the only reason to spend the extra cash for a pre-ban is if you decide you can't live without and adjustable stock, a removable flash hider, or a bayonet lug. Post ban Colts are the same quality as preban Colts, so your paying extra for those 3 features. I'm looking for a preban because I want to build a short barrel rifle, and I can't see paying on a $200 tax stamp and not have a collapsing stock. My only objection to Colt is the ones with the odd sized pivot and takedown pins.
As for quality differences between manufacturers, there's a lot of information out there, some of which (it being the internet and all) is crap. Every manufacturer has built bad rifles - some build far fewer than others. If you head over to M4Carbine.net, and search for a post in the technical section called Comparision Chart of Major AR brands, you'll find an excellent discussion of AR features and benefits - it's worth reading the whole thing. Keep in mind that the chart is based on quality control methods and parts that will result in a statistically more reliable rifle, and that every one of those methods/parts will cost more. The comparison is designed for finding the rifle you would bet your life on, not the most cost effective way to get an AR.
Every one of those listed manufacturers has built good rifles. If your intended use is range only, than the price of a malfunction will probably only be a ruined afternoon, a wait for a warranty return, or the price of new parts, and you may decide you don't have to spend the extra money. Also, everything on the AR is replaceable and upgradeable. The best answer for you might be to buy the rifle you can best afford now, and upgrade as parts break or as your use of the rifle changes.
Of course, with the way availability is now, you may be limited to buying what you can find, instead of what you want. [sad2]