The 12.9" iPP is my first iPad and I love it. I've always had my eye on iPads since the first launch, but I never bought one because I hate buying things that I don't know I'll make very good use of. Mainly because--though I might have found an iPad slightly better in some cases for movies, shows, e-reading, and very occasional casual web browsing and games--I felt that doing those things on my TV, MBP, or iPhone were very acceptable for me already (all except e-reading... I tried out an iPad Mini as a dedicated e-reader for a short stint, but I didn't like that I couldn't palm it comfortably and I returned it, but that's another story). But truly mobile productivity and art were the two holy grails that no device had ever fulfilled for me, until now. Mobile art was obviously lacking because no Apple device had a good, native, pressure-sensitive stylus. And I felt that mobile productivity was lacking because even the lightest Macbook with OSX would never be as light and portable and as versatile and as quick and easy to use and with as great real-life battery life as a tablet with mobile OS/apps; and iPad wasn't productive enough because the screen was too small, it would refresh too often, and until somewhat recently there was no split view (which wouldn't be that useful on such a small screen anyway). But the 12.9" iPP with Smart Cover and Apple Pencil (plus multitasking in iOS) has fixed all that. It's not perfect (mainly I just sometimes wish iOS had more robust file management), but for me it is by far THE best balance of mobility and usefulness for an on-the-go device thus far. And now, besides my work computer, it is my MOST used device--for productivity and art, as well as entertainment. I really never thought I'd see the day that an iPad would become my most used device. Also I heard the addition of the Smart Keyboard really completes the transformation of the iPP into a productivity device--more so than the 9.7" because the 9.7's smaller sized keyboard makes for cramped typing--so I'll probably be trying that out as soon as I get a chance.
And all those entertainment uses for iPad (movies, browsing, etc.) turned out to be superb on the 12.9 as well, as an added bonus. Loving those speakers. Size has not been an issue. But I should note that I knew going into it that holding the 12.9 for long periods, especially lying in bed, would not be pragmatic, so I got a mount and arm to attach to my nightstand. Now I can watch movies/shows and read very comfortably in bed and I don't even have to worry about putting the iPad away before I fall asleep. Also because of the iPP's size and the lack of space on my desk, I got another smaller arm mount to attach to the edge of my desk, so it's great having that play a show while I work. And with Continuity, I can always leave my phone charging on my dresser next to my keys and wallet.
All in all, I could hardly be happier with the iPP. For me, it was 12.9" or bust. But that's just me. If you don't draw much, nor want to do a lot of productivity, and mainly want an entertainment device, and you value better portability over the bigger screen--then I'd say the 9.7" iPP was made for you. Actually, I know what valuing portability over screen size is like. Ironically, I'm a 4" iPhone guy.
Or a foldable iPad.
Start with a Mini. Unfold to 9.7. Further unfold to a 12.9.
Now that would be interesting.
Unfold 5 more times and you'll reach the moon.