This was really an interesting thread to read tonight. I'm thinking of switching to the 10.5. Currently I'm using a 12.9 + mini 4 combination. They're both really great devices, but as far as the 12.9 is comcerned, I just don't feel like I'm really getting as much out of it as I'd like due to the size. I find that I go for my mini much more often as the 12.9 just seems a bit too unwieldy for me to use away from my desk. Plus I rarely if ever need or want to run two apps in split view - that's just not how I tend to work, so the extra screen size for that isn't really a concern for me.
That considered, I really want my main/mobile device (ie, the role that my mini fills now) to have the niceties that the Pro has that the mini doesn't - pencil support, increased storage, and better cameras, to name a few. Plus, I think I'd just rather have one single device that does everything I need instead of two. I think the 10.5 could hit all those needs.
I'm still thinking about it, but the switch seems very tempting to me right now!