I wouldn't bother with an upgrade version (since technically you are not allowed to do an in-place upgrade from the RC to RTM version of 7, which leaves us with the OEM and Retail versions. I created a stickied thread in the Windows 7 section to address questions like you have asked here. Click here
to see that thread. Number 8 in the thread outlines the differences between the OEM and Retail copies of Windows 7. At the end of the day, only you can really decide what's best for you in this situation.