Either will do just fine for application development, and if you don't plan on doing heavy photo or video editing there's no need for the extra processing power of the MacBook Pro.
You might want to consider that the MacBook Air does not have a Retina display like the MacBook Pro, so the Pro display is of higher resolution, and you will have the option of scaling the display resolution to make everything smaller in order to get more screen real estate (but on a 13" display that might make stuff hard to read instead).
MacBook Air is also obviously lighter than the MacBook Pro, so it puts less (if only by a little bit) of a strain on your back and shoulders if you'll be carrying it around a lot.
If neither the optional extra screen real estate or processing power is important to you, and price is also important, go with the MacBook Air. If the price doesn't matter and you don't mind the extra weight, go with MacBook Pro.