iMac. I have an iBook G4 and the 12" screen gets tiring after a while just like the 13" rMBP screen would. It's always a joy coming back to my 20" iMa screen.
iMac for sure. The 13" rMBP doesn't have a dedicated GPU (graphics) as the iMac does. This will make a huge difference in any video rendering or applications (such as video games or video editing). The screen is obviously way bigger, too (more screen real estate).