Depends on how you are going to use. If using mostly in the hand, or on the move, then get the more portable 11”, but if using mostly on desk or lap with keyboard, and with split screen, then the 12.9” will give the better experience.
All depends on individual use which is probably more varied between users on the iPad Pro than any other device.
For me the iPad Pro has entirely replaced a MacBook (I prefer touch screen) as mu main business device, so the 12.9” for me.
I’ve narrowed it down to two options:
11” Silver 512GB WiFi
12.9” Silver 1TB WiFi
Reviews and my experience in a store will hopefully help me decide between the two
12.9 1 TB Easily