This is a tough one... I had the 12.9” original Pro, bought the new 12.9” Pro, and just decided to get the 10.5” instead.
You can’t really go wrong with either at this point. The 12.9” is a thing of beauty, but are you willing to trade off some portability fo that big screen?
I’d say if you’re going to be using the iPad as a primary device, at a desk a lot, the 12.9” is the one to get. Certainly if you’re an artist and are using the pencil, bigger is better.
For typical iPad usage, they 12.9” can be unwieldy. Holding it in bed, on the couch, laying it on a table, it can feel kinda big.
The 10.5” is nicely sized and feels quite a bit bigger than the old 9.7”.
Personally, I also have a Surface Pro (ducks), and that’s my “big tablet”. So I think going to 10.5 was the right move for me.