I know this is an old topic, however I am relatively new to the forums and wanted to share my experience here on the RAM topic as it may help others who have this question.
I have used my girlfriends iPad Pro 9.7 and Shapr3D, the app functions exactly the same as with the iPad Pro 12.9 that I have. However I noticed two things.
The RAM is an issue if you have many other iPad Apps opened up and running in the background. I had to close them in order for Shapr3D to perform the way it does on my iPad Pro 12.9.
The second thing was screen real estate, the iPad Pro 9.7 is great, but when working with Shapr3D I found that I was constantly zooming in when using the smaller iPad Pro.
Neither of these are show stoppers as you can close apps to free up memory and you can zoom in when needed. But if you like to leave things open or maybe forget to close some apps the extra memory helps, and of course more real estate is always nice but not always as convenient.
Hopefully this adds a bit more info for others that may have this question. It was a question I had before making my purchase, which is why I tried my girlfriends small iPad Pro first.