It all depends on what you plan to use your device for. That was the reasoning that I had when I decided to finally step into the game of ipads. The plan was to use it for reviewing notes, such as power points, word docs, and graphs. I was torn between the mini and the air. The mini, I liked for it's small size and portability. The air was great as it had the larger screen, plus it's light. My dad went from and ipad 2 to a mini as he liked the size and portability. At the store I was messing around with both and in the end I decided to go with the air. This was after careful consideration as what would be the best device to do the things I wanted. The air had a nice big screen, so I could review my notes easier than the mini.
If this was a purchase for personal usage, I might have gone with the mini. The goal is to choose the correct device for the purpose you need it for. Both the mini and air are great in their own way. I also like how it's a great way to check email and what not without having to wake up my macbook pro.