previous experience: owned an iPad 2.
1. I have a bad wrist from playing hockey, and the injury still lingers even though it's been close to 6 years since the initial injury. When lying in bed, and holding the iPad for reading, it caused some strain on the wrist (not much, but still) after prolonged periods of time. The weight of the retina mini, though slightly heavier, is still lighter than the Air, and for me, the lighter the better.
2. When the mini originally came out, I wanted one, but decided to wait until it had Retina. That took a lot of willpower, as I felt it was the ideal size for reading.
3. I wanted to be able to use the tablet with one hand. The Air comes close in this regards, but the mini does it that much better from what I've experienced by handling it in retail stores.
4. The price. For the same price, I can get a 32GB mini instead of a 16GB Air. I will never buy a 16GB tablet again (unless it's for my parents) because it just isn't enough space. Part of me wonders if I shouldn't have opted for the 64GB.
With the release of the Air, it definitely gave me some pause for consideration, but I'd say the biggest factors are cost and weight, and the Mini just does it better for me.