I use the first generation iPad Pro 12.9-inch, 128GB, wifi model. I’ve always preferred the A4 paper size for my note-taking. It only made sense to get the 12.9-inch version. It is exactly the size of an A4 paper. It is portable (some will disagree) yet big enough for mind mapping. I can fit a lot of information on a single page. For mind maps, the bigger the screen, the better.
Split view decreases the screen available for each app to use. That is why I didn’t go for the 9.7-inch iPad pro. I use my iPad as both my textbook and notebook simultaneously, especially when I am not at home. Studying on the go is much easier on the bigger screen.
I picked the 128GB simply because I didn’t want the 32GB. I would have preferred 64GB though. I am hoping the bigger storage means I can keep my iPad pro for longer. So far I have only used 32GB for my notes and textbooks.
To minimise distractions, I bought the wifi version. My iPad Pro is primarily a notebook. Paying an extra $300 for LTE is not worth it. It might even make me less productive. Most places I study from (my room, the library) have wifi. An internet connection is readily available most of the time.