The pencil is not a good design, the slippery plastic needs a sleeve and it will roll off a table. There is no way it's worth $100.
Have you seen the ifixit destruction of their pencil? it's really something.
Beside the nice electronics inside, it's also nice to use. It does get a bit less slippery when you use it and then it's really perfect.
I use it with Nebo a lot, but also with GoodNotes and Documents 5/PDF Expert.
I can't imagine life without it. (Well, oke, I did 40 years without, but you get the idea).
9,7 is for making notes like you do on a A5 sized notepad. Nice for to-do's, phone notes, jots etc.
When pressed you hold it horizontal and make longer notes on them, but I do prefer my 12.9 when I'm in college and need to write from 13h00 till 17h00 or 19h00. (Every 50 minutes 10 minutes brake, but around 17h it really gets hard to concentrate).
Writing that long/much is certainly doable on the 9.7 but enjoyable on the 12.9.
Hope this helps.
Just for reference: I've got the 12.9" and 9.7" Pro's. Both with keyboard and pencil. I have three uses:
First with a Goruck GR1. Study books, 32oz water bottle, lunch, both iPads with keyboards and pencils, Bose QC35, AirPods, IFAK/family & knickknacks.
This is what I use when I'm on the road as a consultant. When I arrive at a client I have a thin leather folio that barely fits the 12.9 and 9.7. 9.7 is for the client to read proposals, documents etc. In case they haven't prepaid (most of the time

). 12.9 is for me, typing, annotating etc.
Second use case for the GR1 is when I'm all day in the library / having college. Leather folio stays home, obviously, and there will be several law & text books. The GR1 than just feels and looks like it contains the kitchen sink and all.
Then I've got a small satchel that nicely fits and protects my 9.7" + pencil + keyboard, 16oz water bottle, sweets, Shure se846, if needed a thinner and novels sized study book, small FAIK and some knickknacks.
This is for when I'm just around, with or without family and don't want to haul around the world but want the need and nice to have little things.