I was just asking since everyone keeps talking about 11 and 13" airs if they have regrets not getting a 15" or bigger. The 13" allows for more storage but the frustrating part is the jump from 256gb hd to 512gb on the 15" is $600 and yeah you get dedicated graphics but I have that now with my early 2013 retina. I wanted less heat and more battery since I run windows exclusively. Unfortunately there is not much with windows and accurate displays in the 4.5lb category. Maybe the new dell m3800 will surprise us but who knows.
For what it's worth, I'm less worried about the amount of storage. Even 256gb is enough space to keep the last several months worth of images on board. That 512gb SSD I'm using now will go into a USB3 external enclosure and will provide fast, portable storage when needed. It's very easy to relocate folders in Lightroom to an external drive, so any folders in LR greater than 6 months in age can get moved to the external drive.