Man I hate to say that I’m in this situation as well.
Writing on the iPad Pro with the Pencil is definitely better on the 12.9. I write a lot of notes and there is so much more space for my hands to move around and using it on split screen is perfect. I’m a musician so it’s definitely better to read music sheets on the 12.9 as well. However as a tablet to be used when moving around or on the couch, it’s doable but it gets very unwieldy and uncomfortable after 10 minutes use or so.
Typing with a Smart Keyboard, from my experience of trying them out on the apple store, seems to be equal on both sizes. I prefer typing on the 11 though because of the compact feel to it. It’s almost like typing on a really portable mini laptop. Also I’d need to constantly reach up higher on the 12.9 to navigate the screen with my finger compared to the 11. This is very annoying with Word and Excel.
So I’m thinking the best compromise from both sizes to complete each other is:
12.9” + Apple Pencil: for writing on split screen and reading music sheets
11” + Smart Keyboard: for a more on-the-go typing up messages, emails, browsing the internet, occasional word processing
I know all the problems I listed are very first world problem situations and the solutions are very pricey but hey, as mentioned above, thinking is free 😄