I had a 10.5" but sold it almost immediately after getting the 12".
It's funny - I *almost* went iPad only. I had a trade+cash lined up for my 15" Pro for a 12.9" iPad Pro. After having the 10.5" for a bit, and getting more into my work flow (law student & patent agent) I am sooooo glad I didn't. The iPad is an amazing piece of tech, and there are times that I miss it, but the input of data with your finger as the pointer - even in tandem with the Smart Keyboard - is just too slow to be productive.