I have both machines. Initially bought the USB-C adapter that has HDMI capability (as well as regular USB & a C for charging). Also got an HDMI to VGA adaptor so I can hook up to projectors/monitors either way. In actual practice, never hooked up the rMB to my extra 21 inch display. Found that my rMB has effectively replaced my iPad which means I am taking it everywhere with me in the house, not ever cabling up. Sometimes have the rMB on my lap along with my iMac keyboard/trackpad (the two input devices connected together with 3rd party accessory). Sit in a comfortable exec office chair with me feet up on a couple of pillows on top of another chair so I can stretch out. Then bounce between the two macs on my lap. Also found that I use my iMac much less now that I have the rMB. Really got the rMB to allow me to do a better job on the road with photo editing, but now find that it is my go to device.
PS. 5K Macbook does not support dual display mode, but you could conceivably another display to the iMac with a cable and use Wormhole switch to bounce between the two. My message though is that you might find that the awesome portability of the rMB could change your use case. Not dissing the 5K iMac though as it rocks.