I have the 10.5 and 12.9. I really like them both and can’t decide which one I like more. I wish that I could because it would be cheaper. Lol.
I love the size of the 12.9 for using at home and recommend it if you are after a MacBook or laptop replacement. I take mine to work some days and have no problem doing so. It does become more of a problem if I want to go shopping or something after work though due to its weight along with my handbag and the rest of its contents.
I love the size of the 10.5 for handheld computing and portability. I find it more convenient to take places and for taking photos of receipts for tax time. I also like the 10.5 for reading books (novels), but the 12.9 for magazines, PDFs and comics. Both are great for the games that I play (more the strategy, find or adventure mostly, rather than having to pick up and hold/rotate as with racing car games).
If I was to go on a trip, I’d like the 10.5 in the plane, but the 12.9 at the destination, the 10.5 for taking photos (don’t judge, I’m just saying it would be my preference of the two), but the 12.5 for journaling, the 10.5 for consuming and the 12.5 for creating or consuming. I enjoy watching video on either and browsing web pages or forums on either.
I do all of these things and more and I like to take an iPad with me everywhere, so I’m still torn and not settled on a size. I think I need help.

I do wonder if an 11.5 iPad might be the sweet spot for me.