But I use both, every day, at the same time. They are designed to work together. With universal control, my iPad sits next to my screen at work, and I do some things on the iPad, some on the MacBook. I also use my iPad on its own around the house. It's also incredibly nice for CAD work (Shapr3d) and photo processing (Lightroom).
The iPad is much more resilient to water, food, and dust. I design things in Shapr3d on the iPad and take the iPad to the garage to build them -- a little sawdust does not hurt it and it's easy to clean. It's also much better if you want to read while eating, especially with kids around. The keyboard doesn't get in the way and spilled water won't hurt it either. I use it while cooking and to watch TV while washing dishes.
The other thing the iPad can do that the MBA can't is that it's easily shareable. If you work with others, it's easy to pull up a PDF on an iPad and hand it to someone to look at or even annotate with the pencil. There is no awkward standing over your shoulder or handing off a laptop. Everyone instantly knows how to work with you on the iPad, and to review whatever you want them to review and hand it back.
Overall I still use my MacBook Air more, and it's more capable. You can't do serious coding or development work on the iPad, for example. But it's very much worth having the iPad as well, at least for me.