I have a 2011 13" air and a 2013 15" MBP retina.
For airline travel and use while on a plane the 13 is what I grab every time.
With the 15" I cannot fit the laptop and a drink on the fold down table simultaneously. This is a pain on the early morning flights when you want a cup of coffee while you do some work.
Unless I'm editing video or some other horrendously GPU intensive tasks - the air handles all my office and light photo needs just fine.
For single bag trips - the air, a braincell to carry it, and my aeronaut make a tidy package that is still light and allows you a small bag to carry from the hotel room to the show, or remote office while the larger bag stays in the hotel.
UD