I definitely know its a personal preference. But for me, hearing why people chose the mini or the air really helps me decide. So mini users, why did you choose the mini? Or air users, why did you choose the bigger size?
It is definitely a personal preference, this is my experience. Last year I bought my first iPad and decided on the Air because it was lighter than previous full size iPads and I thought the larger screen would have a better user experience. The Retina Mini didn't have the full color gamet and with the increase weight to get the Retina screen it was not much lighter than the Air.
I ended up using the Air very little. I had wanted my ipad to use out and about but ended up rarely taking it with me. Once it was in a case, it was much heavier. I didn't use at home very much.
I recently bought a Mini 2 refurb (the same model I didn't buy last year), and like it so much better. I am currently on vacation and this the first time I've elected to take an ipad on a trip. During this trip I've had access to my mini, an original Air and the iPad Air 2. I've used all three, and prefer the mini even for just using it in front if the TV, it is easier to hold and use and I think the most enjoyable. It is by far the easiest iDevice I've had for typing on.
I wish I originally bought the Mini last year, I think I would have gotten far, far more use out it than I did the Air.
All that said, I am thinking of returning it and selling my iPhone 5s and getting the iPhone Six Plus. The Mini is so easy to take with me everywhere, I found that I want to do everything on it and wish that it had a phone built in and just have one device. I can't get some of the my third party apps to sync. I've found myself putting down the mini and searching for my iphone so I can set an alarm or do something and have to switch devices so it will be on my phone ensuring that it will be with me. This is already driving me crazy.
My dad has the iPhone Six Plus so I have been playing with that. I think the screen on the Mini is a much better user experience, the best out of all the iDevices for screen size vs portability. However I think it will be easier to manage one portable device instead of two. When I first saw the Six Plus I didn't think the screen was large enough to read my reference PDFs, but when I put some on my father's Six Plus discovered it it is.
I think it comes down to what you want to do. The mini is very portable, and I think is also enjoyable using it in the house.