The Ipad and mac book air are so different.
The mac book air, will give you more computer power, freedom of choice on what to do and how to do it. Air requires tethering or wifi so you loose the choice to have the 3G included like with an Ipad.
The Ipad is more like to enter small info into the Ipad,such as customer requirements, sizes, etc. You won't have access to Sketchup (If you use it), you will have a better/easier way to present images than with the macbook air. (Mac book air can do it,but you have to use the mouse pad, not the screen).
With the Ipad, you might have the keyboard (the one Holzhacker showed) with you or not. It depends if you need it right on the field at that moment, mac book air does not have that option. Without the keyboard you can hold the Ipad and Type, with the Macbook Air will be more cumbersome. The macbook air is more oriented to be in a table/desk the Ipad is oriented so you can hold it.
It is always what do you want to do, just try to imagine yourself in the field.