I'm a bit of a flip-flopper when it comes to technology. I fall in and out of love very quickly. I had the 12.9 when it first came out, but found that it was too big for use in certain circumstances so I sold it and went with the 9.7 pro. The 9.7 was great for awhile but quickly started to feel cramped. So I commandeered my husband's 12.9 (when he's not home), but the 12.9 still not the perfect size for all occasions. Yeah...I'm the Goldie Locks of iPad owners.
When the 10.5 was announced, I was sure that it would be the sweet spot for me. It isn't. It's still a bit too cramped. It's just not enough extra space for me to warrant the purchase. It's all the things reviewers say it is. I mean It. Is. Stunning! But if you're happy with your 9.7 or itching for more space, the 10.5 is not likely to rock your world, at least it didn't rock mine.
I've got a first gen 12.9 coming tomorrow from Swappa (I bought it case I didn't love the 10.5) I've resigned myself to the idea that I'll be living the multi-pad lifestyle. 1st gen 12.9 for some things, 1st gen 9.7 for others. I have to give up my dream of one machine to rule them all.
TL;DR: the 10.5 is too close in size to the 9.7 and too far from the 12.9. 1st Gen 9.7 and 1st gen 12.9 for the win. Long live the multi-pad lifestyle.
Also my ideal range for iPads would be 7.9, 10.1, and 12, but I don't work for Apple and know nothing about the ideal aspect ratio for apple design and apps.