Question probably cannot be answered universally. I find the 12.9 too large and heavy for holding it in 1 hand, it is more a static than a purely mobile device, but then again: a 10.5 with smart keyboard/ cover is as heavy as a 12.9 without, which is partly balanced by the fact that typing on the screen keyboard on a 12.9 is quite comfortable due to it's size.
So it can only be answered individually. At home I naturally sit down in front of my Win10 PC (where I am typing this right now) which is much more verstile in it's functionality as a table can ever be, at work there are PCs as well so no need for a big tablet, and while walking around a big phone is all I would ever want to carry around. So this leaves the 12.9 in a difficult position... I want a new pro to replace my iPAD2 (2011) for photo viewing, but am still undecided which size. Most likely the 10.5, except testing both will convince me of the bigger one. But not very likely.