Running iOS 10 beta 1 on my iPad Air2 - even for a first beta it runs very well. As well as iOS 9.3.3 beta did. I think you should save money and go for the Air 2. Apple Pencil is the only reason I see to upgrade to the Pro.
actually already had an Ipad Air 2 and had to sell and now I will buy another, but did not care about sound, to me it makes no difference, the screen I like the brightness and will not use Apple Pencil, I might use a keyboard but not that Apple, but a Microsoft bending.Apple Pencil isnt even a reason to upgrade imo. The 4 speakers (along with no more vibration) and the glare reduction are the main reasons to upgrade.
My concern would be the lag and slow Ipad with IOS 10 as I said, my use will be to surf the internet, watch videos on Youtube and upload the working documents.