Since the new iPad Mini with retina display is pretty much the same as the iPad Air in every way apart from display size, this is good advice from David Sparks:
For those who are not certain, I’d ask you to look at the ways in which you use your iPad. If you primarily use it for consumption, such as reading websites, books, and other basic computing tasks, you should look closely at the iPad mini. It’s great for those tasks. If on the other hand you’re working on documents, using the on-screen keyboard a lot, and doing tasks that are more content creation focused, you may want to look at the iPad Air.