I have both, as well as a Mac Mini. When I had the old version of a 12.9 (2017– so, pre-design change), I thought the 11 was the best thing since sliced bread. Now that I have the new design in both sizes, though (and now that I don’t travel as much, so don’t need to use two iPads in tandem for work while away from home), I really don’t need the 11 anymore. The only time I prefer 11 is if I am so under the weather that I want to use it completely lying down, and that is pretty rare.
Regarding Magic Keyboard, I probably use it 10% of the time on my 12.9. Love it, glad I have it, because I vastly prefer a real keyboard to any on-screen keyboard.
My use case:
Work— Mac mini for Zoom and viewing/screen share of pdfs on large monitor. 12.9 iPP in a stand for drawing on Zoom whiteboard for my students. 11 sitting nearby on magic keyboard for personal tasks.
Personal— 12.9 in folio for sitting on couch or sitting up in bed playing Civ VI
or browsing or whatever. Also Cricut Design Space. 12.9 on MK for typing anything longer than a text message, including email and journal. I used to choose the 11 for out and about (where I think I will spend time in a waiting room, etc.), but I just upgraded to a 12.9 with LTE and switched the SIM card from my 11 to my 12.9, so I will probably carry the 12.9 unless I really want lighter and more compact— I have been known to ski with an ipad in my little ski backpack on solo days.