I'd say if you want the retina screen on a smaller, easier-to-tote computer, get the 13. But if you want to do anything intensive, whether it's video editing, or gaming, or what-have-you, get the 15 (which is what I did).
The Intel 4000 integrated GPU is seriously weak-sauce for any demanding task. It can't even run Civilization V well, and that game is nearly two years old. The 13" isn't really what you'd call a pro device, since it'll likely get overwhelmed by those demanding "pro" tasks. You can almost think of it as an MBA with a retina screen and the extra hardware needed to drive it.
With the 15, you're paying for power. With the 13, you're paying for form factor. Sure, the 13 is a lesser value in terms of power-per-dollar, but if you don't have those specific needs the 13 becomes an attractive option. That screen really is a joy to use, even if you're just reading the web or your email.