We have an iPad Air and Mini 2 in the house...
Love having both!
The Mini is often by the bedstead used for ebook reading in the evenings. The Air is the "workhorse" for web browsing (or "fun house" or "time waster").
The Mini is also perfect for photography-- right size to hold, right size to view. The Air is too big for that.
I also use the Mini for astronomy far more than the Air -- hence, the user name Saturn--again, the size is right for all sorts of astro apps. It is more intimate.
The Mini falls down, though, in that the text on many web pages, especially Apple's own (i.e., the forums), is too small and rotating doesn't enlarge it enough and pinch-zooming results in text one has to scroll back-and-forth on as it spills off the screen. The Air works much better for that, although, truth be told, Apple's support site is just NOT well matched for the iPad!
The big screen is also nice for readings news apps, weather sites, daily comics, and playing games with a partner where we can both look at the screen more easily. (Although we've found that still can strain the neck, so we end up sometimes bringing up the app, say a word puzzle, on two devices at the same time.)
The Air is also much better for streaming sports, shows.
We're using them, overall, probably equally, but for different purposes. We'd be hard pressed to have only 1 size in the house!
In fact, we've been living with two iPads and one Mini for a couple of years now.
Hope that helps!