I dont really need portability but laptops seem easier to sell and maintain, as well as fix if a repair issue were ever to arise. New battery needed or dead hard drive? no problem, just take it to your local Apple store, and a quick swap is made. It's much easier to carry a rMBP to a local Apple store for repairs than lug around an iMac. Also, I don't have a big room and desktop space is limited, so even if I did go the iMac route, I would have to get the 21.5".
are you serious? one of your considerations is the weight of carrying it back to an apple store for repair?
and good luck repairing the RMBP. if they dont have your specific model (which they wont since it seems like you'll be after a CTO), you'd have to wait for a new machine to actually come in as a replacement since they pretty much can't replace a single part of it.