I had a 12.9 and bought a 9.7 pro to complement it. The 12.9 is nice, works really good for me, much like a Macbook Pro with better sound, more portable, little bit less keyboard, lighter, LTE. 9.7 is "just another iPad" with pencil and a keyboard that works (Bluetooth has been horrible for me). But together?
Holy crap on a cracker!
I'm doing a law study and have all my books digital. I read them on the 9.7, which is a nice pocket sized display. Then I make summaries on my 12.9 using Nebo (freaking brilliant app! Writing does give me more retention, the conversion to typed text does make it a whole lot easier to read back the week before my exams.) Two iPad and I have an endless library and endless notebook. Well, until the battery runs out
When I'm away for work and and don't need to do anything (just a meeting), I take the 9.7, it's light, easy and only difference is less real estate. If I need to work (write text/make appointments/have meetings) I take the 12.9 and sneak the 9.7 with me as well. 12.9 for writing, 9.7 for sharing documents, asking clients to read etc.
Often both my 12.9 and 9.7 are empty around 17h00.
It's an insane investment, two 256Gb LTE iPad Pro's, I'm a bit ashamed, so much money.
But for me it's really working.
YMMV