9to5 Mac poses this question and discusses hardware. My take on this is a little different than the hardware perspective, though it’s completely valid.
The MacBook Air is a entry class laptop built for portability and getting simple things done. It can do more interesting things like Photoshop and Final Cut, but you wouldn’t make a living on an Air with those programs. To make a further distinction on Pro and Air, the retina displays help. But will it last forever? No. Eventually, integrated graphics or discrete graphics will ever beef up or slim down so that it can be in the Air, but by then, the Pro line will be differentiating itself in a different way anyway.
My next laptop, definitely a Mac, would probably be the 13 or 15-inch MacBook Pro with retina. Why? Because, I could use that little extra dose of power and the size and weight difference I feel is negligible enough to accommodate it in my backpack.