Really depends on what your needs are. What sort of tasks will you be doing? What are your mobile needs? Personally, I don't do any real CPU intensive tasks, except for some video encode with Handbrake (light photo and video editing, MS Office, web, mail) and my mobile needs are pretty much consumption (web, mail, videos, some remote use via VNC, reading, and occasional games) so I'd go with the iMac/iPad. Someone who need more computing power on the go may certainly choose otherwise. Also, IMO the Retina Display MBP are all form over function. Don't get me wrong, they look great but is that high of a resolution really necessary? You can get nearly the same specs (save the SSD) in the regular MBP and save $4-600, depending on the model.
I am an iMac and iPad guy myself. When I am out and about it's more consumption than production. Like the previous poster said.
impossible to answer. I might have a answer for you if they get updated september 15. crosses fingers!!!