I'll totally chime into this discussion. I originally went for the larger pro for the pencil support. I used all sorts of styli/styluses for note-taking on my previous iPads while I was at college, and was super excited to use my larger pro as my personal computer and notebook. I studied computer science and now have a job as a software developer, so I can't give up my MacBook Pro (not that I would want to either, it's such a powerful tool).
My day-to-day use of the larger pro consisted of browsing, reading, Twitter, messaging friends, keeping up with email, managing my notes and notebooks (I recommend Microsoft OneNote, if you're curious), and other basic computing needs. I opted for the Wifi only model, the pencil, and a Smart Cover. While I loved the beautiful 12.9 inch screen of the pro for split-screen and video-watching, I found the device a bit too large and heavy for almost everything else I did on my iPad. The large amount of white-space when using the larger pro for reading really rubbed me the wrong way, and made the device uncomfortable unless I at least had an app pinned on the right in compact mode. Overall, I found the device too large for the kind of kick-back activities I enjoyed doing on my iPad. I found that I would still choose my laptop over my larger pro for most activities, as I found it more comfortable and useful.
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When the smaller iPad came out, I instantly knew that I would prefer it over my larger iPad. I got the cellular model along with the Smart Keyboard, and this thing hasn't left my side since. I'm not trying to replace my laptop with an iPad, and I find that the smaller sized iPad is a better compromise between portability and utility than the larger pro, given that I will mostly always have a laptop with me. I was a little unsure and hesitant about the Smart Keyboard; I wondered if I would be worth the money, since I usually carry my MacBook with its awesome keyboard. I've found the keyboard to really enhance my usage, especially given that the it can be used as a cover, and as a stand too.
It's rare that I split the screen 50-50 between 2 apps, most of the time I'm using an app at a time, or I pin a chat in the compact mode. While I still use my laptop as my main productivity machine, I did find that I was able to use my smaller iPad as my personal computer. I enjoy using it for messaging, browsing, reading, doodling, taking notes way more than on my laptop. I also like the smaller size of the device, and how well it feel in the hands -- I think it's the perfect size for kicking back and reading, Apple really did a good job choosing this size from the beginning.
If you intend on only carrying around a single do-all-be-all device, I highly recommend the larger pro. If you're like me, and ditching your laptop is not an option, perhaps check out the smaller pro.