A 12" laptop is going to be difficult for any human being taller than 5 1/2 feet to use comfortably. You're either going to have to slouch badly or crane your neck down. Neither is good for you to do for extended periods of time on an everyday basis.
Besdies that, does it really matter exactly what from usage of your rMB is causing your pain? You need to move up to something more comfortable for you to use. You probably need the 15". Whatever money you might save in staying with a 12" rMB isn't going to be worth whatever injury you cause yourself. Neck injuries don't go away easily (if at all). Neck injuries also can become hand injuries too and then you can't type.
Hi.
Thanks for the input. Just to clarify my personal setup. From work i have a 2013 iMac 27” and 12” rMB as a 2nd computer. At home I had an upgraded 2009 iMac 27” (sold) and now have a 27” display (USB-C) that I dock the rMB 12” into. Setup is OK, but the rMB does not cut it for me personally (need more power for my pet projects) and I do need a private machine so I will get a new laptop... Also might move more into some freelance projects where I need a beefier machine...
Anyways:
rMB 12”: love the silent operation, portability, much to weak for my planned workload (not an option at all)
MBP 13”: 4-core seems powerful enough, still portable enough, screen might be a tad too small for longer non-docked operation
MBP 15”: no need for the dGPU, possibly too large when on the road (also i do have a iPad Pro for casual computer tasks while on the go, super happy with it), pricy, extra 2-core would be cool but not required for my planned workload, extra screen estate would be welcome when not docked
Also, while I have the 27” monitor at home, I like moving around in the house, too... So the internal display is important even if I do not take it on the road daily...
Just a complicated customer I guess ;-) So a trip to the Apple Store is in order...