I find the Air easier to work with if I need a keyboard if, for no other reason, that's more compact to carry around than the iPad and a keyboard, even one that is part of an iPad case. I also often have to access data from one application when using another, and iOS is not at its best when having to do rapid application switching (Command-Tab switching is a lot faster than the double press home, scroll the icons at the bottom and swap).
As well, for editing I find the lack of sandboxing on OSX allows for efficiency enhancing utilities across applications, something you don't get in iOS. I understand the reason for that, but while more secure it also creates issues of its own in terms of usability in a number of situations.
I prefer the iPad for totally single application focused situations where little or no input is required. I use my iPad to hold copies of manuals when I'm doing a full day continuing education presentation for CPAs. A MacBook Aire can't really fill in very easily for that usage.
I see them each as tools appropriate for different jobs.