Assuming Air Haswell since both 2013 ?
I don't know about others, but I would prefer new versus old on Air and not sure since both airs were 2013 which you are speaking about without looking and I won't, but as far as better, processing, and connections, and screen obviously the Pro, but the Retina's be rendition is on the 15 inch, so if you lean toward the PRo you would be best served with the bigger one and getting quad, and retina much more significant, and speed, and other facets. I wanted to get the new air, and I am sales with lots of flying so for me the size and transportability was key, but then they released without the retina, but a week later released the mini with retina , thus a signal that they will be doing the Air, but couldn't in time, so I am waiting for Xmas , and expect an updated Air with the Retina display in time to buy for Xmas season.
The Air just got allot clearer with the new Apple Retina Display under your Xmas Tree , I can see it now.
But if must get now, it depends if size #1 otherwise the Pro, and once there, the 15 is worth the difference of 500 all day long, and you also can put 8 more ram which using I7 now isn't huge, down the road will be, and you can have dedicated graphics core which is all the difference also.
Good luck