From Apple's tech specs, the only differences I can see between the Retina Mini and the Air is the weight and size of the screen (the resolution is the same). So the extra $100 is for bigger heavier pixels?
Basically, yes. I personally will be buying the Air because I like my older iPad's size vs my iPhone. The mini is a great product, but it's just a little too small for me to fit the bedroom/travel entertainment category that I'm looking for. Plus, playing board and light-3d games on the 10" screen is really a joy.
More materials and more battery. That's what you are paying the extra for. The larger device is better for collaboration and creative use whereas the smaller one is better for casual use and reading, but both are equally capable when it comes to choice of software.