I manage the sales dept at an Apple Specialist store.
Just like the question that was posted recently "which camera should I get", there is no easy answer.
Do you NEED a portable? How much local stage do you need? Do you have a budget? What do you mainly use your computer for? Are you into photography? Do you do a lot of reading (like this forum :-) )? How long do you plan on keeping the new computer for?
You get more bang for your buck from an iMac. The big thing is that all of Apple's laptops (short of the mid 2012 13") use flash storage, which comes at a premium. However, you may be able to use your aging iMac to host your files. You could also get a newer iMac and use your 17" MBP when you actually need a laptop.
I personally like my iMac and I use my iPad as a portable. I wish I had an iMac with Retina display and I would get a fusion drive or flash based storage.
Edited by Bill D, 15 June 2016 - 01:14 AM.