I've got both, plan to keep both.
I'm a front-end developer, and am at my dev laptop all day long. The iPad(s) fill basically the rest of my time, be it sketching, painting, reading, movies, tutorials, textbooks, gaming, etc. I prefer the larger at home unless I'm reading a novel, then the smaller one excels (esp w/TrueTone).
The big size was nice at work for split screen while in meetings and taking notes, but it's just too bulky (for me) for everyday carry, so I prefer to carry the 9.7" to work but still have the same abilities as the 12.9". With Evernote, Notes, Box, etc -- I have access to all my files easily on both, work and fun.
It's a bit extravagant to me to own both, but I work hard, make good money, and don't buy other luxuries -- Apple is my "sweet spot" for fun/useful tech that I use for hours every day.