I don't have any experience with these parts but I have been looking at them recently because I intend to replace my upper and lower ball joints soon as well. I too have found K7424 listed as the Moog upper. However, I did a quick search and found this article that I believe explains it.
Basically, it sounds like the "R series" is Moog's cheaper line. I guess they got tired of losing sales to the Advance Auto and Autozone brands and wanted to get in on the cheaper end of things. My thoughts are that I want to buy Moog for the quality to begin with so I'm going to stick with the original Moog parts line. To each his own though. FYI, the cheapest place I have found parts are on Amazon. They have the K7424 for $63 and the RK7424 for $47. Also, if you sign up for Amazon Prime you can get free two-day shipping on all Amazon purchases for a month I believe it is. If you don't have a need for the Prime service, you can easily cancel it within the trial period. It's free to sign up and free if you cancel within the allotted trial. Hope this helps!