After spending several days with the 10.5” iPad Pro, I decided to return it for the new 12.9” model. Here’s my reasons: Most of my working time (95%+) is spent at a desk. So for me, portability isn’t an issue.
When I use the iPad Pro it’s primarily for sketching prototypes for web interfaces, notes from meetings, etc. For these situations a larger screen is more important. It’s basically the size of a letter (or A4) piece of paper and means I don’t have to do as much zooming.
I really wanted to love the 10.5” size, and for many people I’m sure it will work well. However, as Rene Ritchie pointed out, if you want a laptop replacement – and yes, I think it’s possible that this could replace a laptop for many people – a larger screen with full sized split views makes sense.
If it’s more important to have portability, then you may want to look at the smaller size. At the end of the day I made the decision, that as a designer, that screen size was more important than anything else.