well, 1st off, welcome to the site!
second, READ, READ, and READ some more; and try out your 'search' feature
most quality, forged, stock AR uppers (and sometimes lowers) are made by the same company and simply marketed by the big AR brands; while i cant verify off hand who makes what, i can say that the only difference w/ the listed brands you have is probably going to be on the finish--they are all qulaity, top notch AR uppers recivers
delton i believe gets the same recivers as DPMS's; not sure on armalite's, but i think thier upper recievers come from the same dist as busy and RRA...cerro forge is a popular maker
www.ar15.com/forums/topic.html?b=3&f=12&t=223495www.ar15.com/forums/topic.html?b=3&f=123&t=297440usually, you are paying for the name and logo, so yes, its mostly a marketing thing--for example, i dont see how a upper or AR from DSA, LeBaer, or Wilson Combat is worth $1K more than anyother brand out there, other than the NIB finish, name, and logo
imo, as long as it works up to spec and there is nothing wrong w/ it (which would apply to all the brands you listed, except EGW as i dont know anything about em making AR parts), i would pick the $90 one from del-ton, or better yet, the ones for ~$100 by CMT/stag
you look for key words like: "aluminum, forged etc" from a reputable brand (ABCDRS); sometimes, be warry of those who use the words "mil-spec" or "USA made" all the time....while all quality made material should be 'mil-spec' or better, there are companies out there who simply use these words as thier marketing scheme, and it dont mean a hill of beans on thier quality, longevity, or functionality (hesse/vulcan is one brand i can think of to avoid b/c of this)