At the time of this post, I am only a owner of a 1 day old iPad pro 10.5, just installed Shapr3D last night. To me, it is merely good enough as I simply want to try some simple modelling on Shapr3D when I need for some office's project while our Engineers uses the heavyweight Solidworks on PC for the whole projects.
I do love the big screen of the 12.9 but I find it kind of Big if I need to carry to office everyday. Do note the overall weight too if you intend to add in the Smart Keyboard. The 10.5 is kind of light in the weight, it does weigh a bit after adding in the Smart Keyboard. Thus, I am sure the 12.9 will be heavier.
As such, it really up to you, which was your preference and how you use it.