Yah the Air 2 Gpu really has the extra oomph to power the high res display were as the A8 Gpu works well on the iPhone resolution but is limiting on the iPad Mini. Now in normal day to day use and such I don't think you'd really notice the difference as the Mini 4 is still rather powerful it's not like the Air 2 will get more iOS updates as Apple has probably just lumped all A8 devices into the same support bracket. So really it comes down to what you want size wise and how important is the overall performance of the device. For me I'd never buy a Mini just becaise if I'm on my iPad I'm either working or consuming content and I don't want anything smaller than 9.7inches. Also overtime with iOS updates you may see the Air 2 continuing to perform rather smoothly were as the Mini 4 will be more prone to the typical iOS UI stutters. Gotta say I'm glad I hopped onto the Air 2 back in 2014 as it seems like it's going to have some real longevity as far as smooth performance goes with iOS updatesiPad mini 4 has a regular A8 chip. iPad Air 2 has a Tri-core A8X chip with (more importantly) an enhanced GPU that actually can handle the blurring effects and animations perfectly smoothly. For single core, iPad Air 2 has about a 15% edge in performance. For multi core, a whopping 60% edge.
The GPU is DRAMATICALLY better with the Air 2. I was using an iPad mini 4 in store and was not impressed. A lot of janky, slow, choppy animations JUST like my iPad mini 2. iPad Air 2? NONE of these issues. At all. Butter smooth, super snappy, perfect.
A lot of other things such as L2 Cache, memory performance/bandwidth, etc etc are also better on the Air 2.