After playing around with both 2017 models (12.9 and 10.5) in a store, and although trying hard to like the 12.9 because of it's basically gorgeous & huge display, I finally ordered the 10.5. Now 10.5 feels much closer in display size to the 9.7 than to the 12,9 (which of course is also mathematically true). I just did not find the 12.9 a very good handholdable device. Possible yes, comfortable, no.
As the 10.5 is about the same weight as the 9.7 and both clearly lighter than my old iPAD 2 (2012), it is the truly mobile tablet. After using it for a little while, the 9.7 display now seems clearly smaller than the 10.5, almost smallish.
I also ordered the pencil and plan to explore it's capabilities with iOS 11 once it is out, if later I still lust for a big display, I may still buy the 12.9 for special purposes like higher rez photography etc.