Hobbyist here. I have a 2018 12.9”. Screen size was a big factor for me. To me the 11” is fine for content consumption, reading, web, etc., but for workflow, the added screen real estate is a huge win. I use my IPP for most of my initial editing on the go, and only really go to my laptop (16” MBP) if I need to do stitching and large exports.
In the Mx series ipads, the other added benefit of the larger screen is the screen quality. That goes a long way in editing photos.
Last, in hearing the rumors of 14” and even 16” IPP’s in the future, I would easily upgrade to those when available. It may be the same footprint as my MBP at that point, but it will be much thinner and lighter, so not really a big deal to me.