The answer depends on the types of apps you are developing. Apps that require some of the specialized hardware on the iPad (accelerometer, GPS, compass, etc…) you won't be able to effectively test in the simulator, so the size of the MBA screen may be fine. However for simple apps that do not require any specialized hardware you may find running in the simulator at 50% a bit too cramped, so you may want to opt for a MacBook Pro with Retina display. Running in the simulator has a much faster turn around than running on a device, especially on a Retina MacBook Pro which is as fast as my 2007 Mac Pro.
Personally, I bought a tricked out MacBook Air for my hobby iOS development. I feel I can get away with it because most of my iPad development is on the device. If I used the simulator more I'd move up to a Retina MacBook Pro. I also get more time to play with my stuff because I am willing to take the MacBook Air with me almost anywhere, as opposed to my 15" MacBook Pro which pretty much sat on my desk.