It's largely a matter of personal preference. No one can tell you which will be best for you. I personally use and adore the retina mini, but that doesn't mean it will be right for you. See if you can get your hands on an air and a mini (a friend's or a demo unit at a store) and play with them for a while. You may find one of them calling out to you.
Some factors that might make a difference:
Do you typically wear a suit jacket or carry a purse?
The mini slips comfortably into a typical suit jacket pocket, so if you wear one or carry a purse, it can be incredibly portable.
Do you want a compact device that's unobtrusive in a crowded line or public place and that you can take everywhere?
For me the mini is the perfect form factor. I can take it out and hold it one hand in a shopping mall with a bag in the other to use the GPS, or hold it with one hand and copy problems out of it onto the chalkboard with another in a classroom. That said, if you want a device primarily for at home use, the Air might be better.
Do you want to use the device to watch movies? To read comics? To read sheet music?
In my opinion, visual media look better on a larger screen, and the improved color range on the air may make a difference (not everyone notices it). I've heard several people say the small screen of the mini is too cramped for sheet music.
Do you want to use the device to read pdf/textbooks?
The mini is great for kindle books and the like, as they re-paginate the screen as needed and you can choose whatever font size you like. Pdf files and textbooks may not be so flexible, meaning the screen may be a bit small for these. That said, my primary use for the mini is reading pdf documents, and while it's a bit small, the portability of the device is more than worth it to me. But then I do have good close up vision.