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No one really has to imagine how it would have been on iOS 11. A great many iPad Air 2 owners have upgraded to 11.*. Some have complained. Others, like me, have no complaints at all. There is a ton of discussion on MR.

My old iPhone 6, on the other hand . . .

This. iPad Air 2 is Running just fine while the iPhone 6 performance has become a plain disaster.
 
I have an iPad Air 2 running the latest iOS and it is doing just fine. I use it for entertainment and productivity. While it may feel slow when it is running next to an iPad pro, i don't notice since i don't have both devices in front of me (nor would any other average consumer). On the other hand, i do notice the slow nature (not a disaster) of my iPhone 6 running the latest iOS. Since I have taken care of both devices and keep them in cases, neither has a scratch or dent on them.
Apple designs products that last a long time and saving money is not a crime. 6 in 10 Americans don't have 500 dollars saved.
 
Listen, I have the 2017 and my girlfriend has the Pro...there's literally no difference other than I paid 300 and she paid 700.
 
Another thing to check out and consider is... speaker vibration. The Air 2 has a nasty habit of vibrating the whole unit. Not a deal-breaker but certainly something to be evaluated.
Not everyone gets easily annoyed with Vibrating. Though majority of European and Asians kinda like the vibrating speaker inside the screen even on Pro As well as it massage their hand. iPad 2017/2018 it's funny that it looks like an OLD iPad Air 1 with plastic dull screen, if you tried to use it on brighter environments it will Strain your eyes.

iPad 2017 lacks Fully Laminated Display
iPad 2017 Has UGLY Black fat border between the Bezel, and screen.
iPad 2017 Heavier, Thicker and 7.5 MM vs 6.1 MM on Air 2/Pro's
iPad 2017 STILL 2GB RAM is just like iPad Air 2/Mini 4 came from 2014/2015

Like seriously? i am sure that 2GB Ram is starting to go downhill once iOS 12 Comes, though the weakest 2GB ram that came from iPad Mini 4/Air 2 is already getting slower all together with iPhone 6 Series.. Next would be the A9 will slow on iOS 12.

To answer your question:
If you wanted an iPad that could last until 2020 get the iPad 2017 it will absolutely get iOS 13 but it will be laggy.
If you wanted an iPad with Better screen, anti reflective, but only last until iOS 12 at 2019 iPad Air 2 is there for you.
 
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