My wife has been wanting an iPad Pro to possibly replace her laptop, but she was waiting for the second generation, as you would expect so late in the lifecycle of the first one. When the new models were announced, I ordered her a 12.9" iPad Pro and an Apple Pencil.
When it showed up, I played with it a bit, of course. Then I went and bought a 10.5" for myself, along with Pencil, and a Smart Keyboard (aka keyboard cover) for each of us.
In which I try the 10.5"
In some ways, the 10.5" suits me better. It's more maneuverable, and I can thumb type on it fairly easily, which was a bit of a surprise. If using the iPad to do any reading, the 12.9" can be a bit awkward. There is still plenty of room on the 10.5" screen for drawing, and typing isn't bad.
In which I change my mind
However, after using the 10.5" iPad for the weekend, I went and exchanged it for the 12.9". Why?
I quickly learned that what might be one the primary use cases for me with the iPad Pro is to use split screen mode to have reading material on the left, and a notepad for handwritten notes on the right. I also have dabbled a little bit in learning to actually draw, and while I don't know if I'll keep with it, it's nice to have a website with instructions on the left, and a practice notepad on the right.
This is certainly something that works on the 10.5", but once you cut the screen in half and limit your writing and drawing to one half of the screen, things start to get a little cramped.
When I returned the 10.5", they didn't have a keyboard cover in stock. Not knowing if my wife and I would each really use it all that much, I decided to get a normal cover and test out the keyboard cover I'd already purchased.
A tale of two keyboard covers
When using the 10.5" iPad Pro, I kept the keyboard cover on all the time. It was pretty light weight, and there were enough magnets that if you flipped it to the back of the iPad when not in use, it would hold in place fairly well. The keyboard cover also made a surprisingly good stand. Even if you weren't using the keyboard, flipping it open into a stand worked great. You can do this and have the keyboard tucked in behind, so it's not sticking out and getting in the way. There are a lot of magnets in the 10.5" keyboard cover that help it to sit very solidly in the various positions.
The 12.9" keyboard cover isn't nearly as nice. When flipping the keyboard cover around behind the iPad when not in use, it doesn't even remotely stay in place as well as the one on the 10.5" iPad. It kind of seems like they realized the limitations when they came out with the 10.5", design that cover to work better (since they were making a new one anyway), and ignored the 12.9" cover. It could just be that with the larger and heavier keyboard, it just doesn't stay in place as well.
The weight was also an issue. I assume it's just because they fit a larger keyboard in because of the larger amount of available space, but it felt like it added a lot more weight proportionally than the 10.5" did.
It also didn't seem nearly as sturdy as a stand as the one on the 10.5", but that could partly just be the increased size of the iPad and keyboard making it awkward for some surfaces.
Since my wife didn't think she'd really use it much, we returned the 12.9" keyboard cover and just went with Smart Covers on both iPads.
Onscreen keyboard addendum
The extra room on the 12.9" screen lends itself to a larger keyboard, so they patterned it after a laptop size keyboard, more or less. I'm not convinced this is the right move, especially in portrait mode. In iOS 11, at least, I felt like with the 10.5" keyboard (sans number row, since the numbers are accessible with Key Flicks) gave me larger buttons, which is very handy when typing onscreen. This could be entirely my imagination, as I never set up both sizes side-by-side to see how they compared directly.
Different people have different needs, so by no means should you take this as a guarantee that you will come up with the same answers I did. I just figured I'd put my story out there in case my experience helps anyone else.