I've used both, and in my experience: the 10.5-inch model is the one that makes the most sense.
The 12.9 is for artists, or those who really do intend to use the iPad as a pseudo-laptop virtually all the time. But using it just as a plain tablet? It's like trying to manipulate a dinner tray... it's simply too big.
Meanwhile, the 10.5 still feels like a mobile tablet. It's much more portable, to the point where you can (usually) thumb-type on it in portrait mode. And it's much lighter, so it's easier to carry in your hands instead of resting on your lap. Even if you do intend to use it as a laptop, the slightly larger screen versus the 9.7-inchers actually makes the new Smart Keyboard considerably easier to type on than its predecessor.
I have both a 10.5-inch iPad Pro and a MacBook Pro, and if I didn't need the MBP for work, I'd honestly use the iPad as my portable machine -- it's easier to tote around!