Air has noticeably better battery
Mini 4 has noticeably better display
Sound is similar with neither having the Air 2 vibration issue.
Just a personal choice on form factor, imo.
I kinda am wanting the mini 4 cause I find my air 1 to be awkward in some situations. Like especially when bringing it out of the house. The mini is still an iPad, and can still do more than my iPhone 6, but in a more convenient way than the air I think.Im in the exact same situation. I have the Ipad air 1 but Ive been using it less and less. I usually use it when laying down on my bed or the couch and its not really comfortable to use. Whether im typing or just browsing internet, i cant hold it up with one hand or type with two hands easily. I have pretty big hands too and I still feel some discomfort. Im considering a mini the next time I upgrade. A bigger screen isnt really a big deal to me either.
Only thing I dont like is that the mini 4 is less powerful than the Air 2, I may just wait for the mini 5 next fall lol
It would be hard for the Mini 4 to have a better display than the Air. It would great if it were merely as good. Check this evaluation of the Air, Air 2 and Mini 2 displays: http://www.displaymate.com/iPad6_ShootOut.htm