I think this debate is most dependent on what you'll be using the laptop for. If you're a student, I think portability becomes more important than most other factors. If you're in the business world, will you be using your computer with an external monitor, etc.? All of that is relevant to the convo.
Personally, I use my 15'' for work and my 12'' for leisure. I hook my 15'' to a monitor and use it as a desktop most days, however when traveling I really do enjoy using the notebook's 15'' screen. I work 8+ hour days using my laptop while traveling, so having the easier to read screen really comes in handy. It's also way better for showing a coworker a file, etc.
I really can't imagine using the 12'' for work-travel purposes...I find myself bending over to read when using my 12'' at a desk for a long time. The device is so small you have a tendency to want to get closer to the screen. I think it'd give me and others neck pain for long term usage. This is why I use my 12'' on the couch or in bed as a leisure, light-lifting device.
It all depends how tall you are, what you're accustomed to, etc. But I think if you are tempted by the rMB OP, go buy one. You have 14 days to decide if it fits your needs or not.