I really don't understand why the hell these Late-2012 iMacs are having such cosmetic issues, when all of the other years never really seemed to have that problem. It makes me think that Apple either has the people building them rushing their asses off, and sacrificing QC for production. I have never received a computer with any kind of cosmetic damage.
At the same time, if the display was perfect, with no yellow tint, backlight bleed or image retention, I would probably just keep the one with the scratch, if the scratch is on the back of the computer. BUT, at the same time, you have to figure that this little scratch, while it may not be as big of a deal to some people, considering supply and demand and all the other problems. A year from now when all this is said and done, when you go to resell the iMac, those minor imperfections are DEFINITELY going to affect sales and the price people will pay. They wont be willing to just "accept" some minor cosmetic problems, because they aren't dealing with what we are dealing with right now. Therefor, they'll just find someone else selling theirs, which is perfect.