There's no hard, fast, objective rules for AR15-related equipment. Just like everything in life. You have vendors and dealers who are pushing their own agenda, you have deliberate misinformation, big egos, garden-variety ignorance, and situations that are simply unknown.

Your best bet is to get your hip-waders out, read your eyes out on ar15.com, weaponevolution.com, m4carbine.net, and come to your own conclusions. There isn't one easy answer.

My take is this: Caveat emptor. You get what you pay for, except when you don't. Most suppliers are average. Some are better, some are worse. The field is probably tighter than in some other markets, because of the existence and presumed use of a standard mil-spec/tech data package, and the acceptance and standardization of certain manufacturing and QC technologies.

Very personal take: If you avoid Olympic and Vulcan, you won't go too wrong. Other than that, look at brands and models on a component-by-component basis. Some companies do better than others on some parts, and there's some parts where it flat-out doesn't matter who the maker is.