I've been playing with the 10.5 coming from my 12.9. It feels a LOT smaller. I'm not yet convinced if I want to keep it or not. The display is really nice, and it is fast and responsive. I'm just not sure if I can get over the size of the apps in split screen mode.
I'm I the same boat. I have a first gen 12.9 and have loved it since the day I got it. The size was a little much at times like reading or watching movies in bed or whatever but not horrible. The new stuff in the 10.5 got my interest and I happened to find a really great deal on a return at Best Buy so I took the plunge. Spending a couple days using both side by side but honestly, I keep leaning back to the 12.9.
I'm not going to rehash the spec differences here since there are a million posts about that but to me, even though I see some difference, it's not earth shattering. The Apple Pencil is a bit smoother but it's not like it was terrible of the 12.9 to begin with and True Tone and ProMotion is nice the first two seconds you look for it then it just gets forgotten.
I think one thing that bothers me more than anything is that everything still seems so 'small' on the 10.5. Honestly, it still feels like the older 9.7 to use. The smaller (full size) keyboard, truncated Split View apps, etc... still makes it seem like a lot of software compromises for the sake of size. The weight difference is only half a pound so realistically it's not much.
Wondering if anyone else is seeing the same real-world experiences. I mean, you can compare specs all day and oohhhh and ahhhh over that but the truth is in using the devices all day and seeing how the differences really affect your work and use. I think I'm sticking with my 12.9 gen one and waiting to see how much more amazing iOS 11 makes use of that still beautiful screen.