It's more easier than that. It depends on your tastes. Right now, apple has rop notch pipeline in all ipad sizes.
-iPad air 2 is a beast, don't think about absence of updates this year. It's perfect now(only has the problem with vibrations at high volume), and run flawlessly with ios 9/9.1.
-ipad mini 4 is finally(if the news are true and it has 2gb of ram and the new full-laminated display has the same colour gamut than air 2), the perfect ipad mini...don't think about 100 mhz more or processor names...if ipad mini 4's a8 has 2gb ram like new apple tv's one, is different that iphone 6/6 plus a8. A8 is not always the same processor with the same specks. It's only marketing. Apple could have named ipad mini 4 processor, a8x but the prefer to sell more ipad air 2 because it should have much higher profit margin....
-ipad pro has a9x. Ok. Brutal processor, with much better specs but... the ipad pro screen has more than 5 million pixels, and it has the force technology that needs the pencil to run... SO A LOT MORE REQUIREMENTS ...maybe the new oxide tft display has decoloration or worse gamut, maybe run worse than ipad air 2 or ipad mini 4(both with last year technology) ..
Think about. Apple has the perfect specs for each size right now(I repeat, if 2gb ram and screen quality of ipad mini 4 is confirmed). So, choose a size, depending on your tastes, and do not spend too much time thinking about specs or future-proof. Enjoy your choice and be happy!