I don’t know about the person you’re replying to but I had both sizes in a previous gen (2017, so I had 10.5 and 12.9).
A majority of the time I preferred the 10.5 for day to day use like browsing around the internet while laying around, watching videos, etc and I also used it when I would go out because it was such a nice size to get a “close enough to a laptop” type of experience while having a much smaller footprint, which was great when on flights, or just chilling at a shop, or whatever. Overall the 10.5 was my daily go-to.
However, the 12.9 was VASTLY better in a few notable areas. I used it for gigging and sheet music (shoutout to forScore, best app for this) and it was so much easier to read while in a rehearsal or on a stage than the smaller iPad. Also taking notes by hand and drawing on a 12.9 is far better than the smaller iPad because the 12.9 was very close to the size of a standard sheet of paper, but the 10.5 was closer to something like a small diary or something. I do software development/architecture and when in meetings the 12.9 acted like a nice whiteboard that I could pass around to everyone and it felt way less cramped than the smaller iPad. Furthermore, if I was doing some heavy research but was wanting an iPad (maybe for diagramming or writing by hand - always preferred writing by hand when possible) then split-screen multi-tasking is vastly better on the 12.9.
I guess put shortly, the 10.5 was more enjoyable for fun things and the 12.9 was more enjoyable for work things. The 10.5 did always feel like a nice breath of fresh air after having used the 12.9 for extended periods of time because it’s so much more airy and nimble, the 12.9 is clunky.
All of this being said, this was all on 2017 iPads. The modern 11-inch iPads have almost the same footprint as the old 10.5s but the modern 12.9s have a significantly smaller footprint than 2017 12.9s, so I’m thinking that I might prefer a modern 12.9 for most of everything. I’m currently using an M1 11-inch, but seriously considering returning it for a 12.9.