It’s tough to say. The portability of the Air is absolutely amazing, but I find the 13″ screen can be a little small at times. I think you should get a computer that is best for the *majority* of your situation. So if you are designing at home for 90% of the time, get a 13″ Air and hook it up to a 27″ monitor.
If you are on the road more than you are at home, then perhaps a 15″ is more up your alley.