I have both and use both on a daily basis. The Mini is the one which goes with me when I leave the house; it fits very nicely into purses or bags. The iPad Air is the one which goes out on the deck with me in warmer weather. When the Mini is on the charger, the iPad Air goes out-and-about with me. I am appreciating its lighter weight and smaller dimensions -- it's wonderful to carry around, unlike the earlier, heavier iPads.
I use the Mini for catching up with forums, reading the news, checking the weather, playing Solitaire or some other game occasionally, and of course for email. Occasionally I will read a book on it, too. Sometimes I watch an episode of a TV show on it but usually I do that on the iPad Air. The Air is my go-to iPad for watching movies, videos and TV shows, and of course does all of the other things as well. I have both set up pretty much identically, which makes it easy to pick up one or the other and be able to do the same things on either.
Not for a moment do I regret having purchased both!