There's only so far you can go with an ar15. The best aluminum, the best materials for the barrel, bcg, etc, the ergonomics of the charging handle, stock, grip, the functionality of the guard, the reliability of the working parts... After that, you are paying for name and looks. Some that revolutionized design from the old Vietnam look to today's tacticool cashed in years ago and are trying to hold onto their status today, even though as someone mentioned earlier, many other companies have caught up in functionality and design.
If you're set on emptying your bank account and maxing out your credit card, just become a H&K fan boy. They'll take care of you!
you obviously don't own one or have never build an AR