The main difference between the 10.5" and the 9.7" when it comes to split screen is that the new 10.5" will show "Regular size" class UI in the 80/20 view on the larger app, whereas the 9.7" Pro, 2017 9.7" iPad and 7.9" iPad mini show the "Compact size" class UI in all forms of split screen multitasking.
Taken independently, this is an improvement over the 9.7" iPad Pro, and a good one. However, in context, I can't help but be disappointed. It is a shame that Apple has chosen to differentiate in this way between the two iPad Pros, even though it is a very "Apple" way to nudge towards the larger, more expensive device. I'm still buying the 10.5" though. I couldn't stomach the 12.9" because it was bigger and heavier than my rMB. (Although this attitude might need to be adjusted if the 10.5" actually works as a real contender to replace the rMB. Then the 12.9" might make more sense, but seems to move away from the holy grail of a "single device" - feel almost like I would have to have an iPad mini around as my "iPad" if I had a 12.9" again...)
Now, "Compact size class" doesn't actually mean iPhone UI - it means a UI optimised for resolutions smaller than "Regular size class". The iPhone Plus actually has a horizontal "Regular size class" classification when in landscape mode (why Mail.app for example shows your list of messages in a separate left column when in landscape) There are several differences in almost all first party apps (and lots of third party ones too) between iPhone UI and "Compact size class" UI's, but they are similar, and will adapt depending on the point resolution the app is given to work with.
In any case, I'm still excited to get my hands on the 10.5" and immediately put the iOS 11 beta on it to put it through its paces as much as possible within the 14 day window. The 10.5" ASK arrived today. I have a thread where I compare it to the rMB keyboard.