Unless you prefer one size or the other (and it sounds like you don't have a clear preference, otherwhise you wouldn't be asking), it seems to come down to this:
- Air 2 is more powerful
- Mini 4 has better battery life
Personally, I'd go for the Mini 4, because I mainly use my tablets for reading, browsing etc., so more power is less important than better battery life.
- Air 2 is more powerful
- Air 2 has a much bigger screen
- Depending on the test, Air 2 may have much better battery life, or else slightly worse battery life
- mini 4 is more portable
- mini 4 is cheaper
For video playback, the Air 2 has 35% (!) better battery life:
For web browsing, the mini 4 has 6% better battery life:
Personally, I'd go for the Air 2, because I mainly use my tablets for browsing, video playback, etc., so the much bigger screen size and video playback battery life are more important than a small improvement in portability; given the light weight of the Air 2, it is already quite portable. It just doesn't fit in an average purse, but I don't carry a purse.
Plus I purchased the Air 2 for CAD$100 off retail when it went on sale back in September, just before the mini 4 was announced. For $100 off retail, the Air 2 was a no-brainer.
BTW, here are some of the tablets and phones we own:
iPad Air 2 (mine)
iPad Air 2 (wife's)
iPad 2
iPhone 6s
iPhone 5s
Nexus 7
The Nexus 7 is never used. Part of it is it is too slow and I prefer the software on iOS, but part of it is that the screen is just too small for my liking. It's more portable than the Air 2, but it's not as if it just slides into my pants pocket like an iPhone does, so if I take it with me I still need to carry it separately in its own case or inside my briefcase (despite the fact the Nexus 7 is smaller than the mini 4). And if I'm going to do that, I may as well just take my iPad Air 2, because the Air 2 is actually much thinner than the Nexus 7. Inside my briefcase, the width and height don't matter, since both the mini 4 and Air 2 easily fit with regards to those dimensions. The more important measurement here is the thickness, and the Air 2 and mini 4 are the same thickness.