Long time 11" user, and just bought and returned a 12.9" after a week or so. It was pretty confusing trying to figure out if I wanted to keep it or not.
I just bought both due to the amazing M1 sales going on, so I'm chiming in as well!
Firstly, the 12.9 screen and size were both breathtaking. I really want the 11" to get mini LED. But the novelty of having such a big and beautiful screen on a tablet does wear off. And once the novelty wore off, I started to run into issues.
Having finally gotten a chance to compare the mini LED with the regular display of the 11", the mini LED really does pop with the better black levels and HDR brightness. I like it a lot and it's annoying the 11" doesn't have this yet.
The aspect ratio and size is cumbersome. I like having a bigger screen, but it being such a tall 4:3 aspect ratio means there is a lot of size and weight that's going into portions of the screen that are somewhat unnecessary. The bigger black bars mean that while the 12.9" still displays video content meaningfully bigger, it's not enough of an upgrade to justify the extra size and weight.
Every aspect ratio has its tradeoffs, so this doesn't bother me. I actually love the 4:3 aspect ratio. I feel the poor utilization of the screen is mostly due to the stagnant multitasking the iPad has been stuck with for so long. If I could just place windows where I wanted them, it wouldn't be an issue.
I love the magic keyboard on the 11", but on the 12.9" I found it a bit annoying. The key travel is a bit too deep, and the typing is actually quite loud compared to the 11" magic keyboard and the 14" MacBook Pro keyboard. I normally don't complain about these sorts of things, but I couldn't stop noticing it.
I'm still waiting on my 12.9" magic keyboard, but I do like the typing experience on the 11".
I also found that the weight of the 12.9" magic keyboard just put the 12.9" too far in the size and weight vs capability and niceness ratio. With a smart folio, I kinda understand it, but once the thing gets to 3lbs with the keyboard, you're entering a size and weight (and price) class that just commands more capability. 11" with MK, it's like, I can't believe I can do so much on this tiny little device. 12.9" with MK, it's like, I feel like I should be able to do more with a device this large and expensive.
Finally, I found it just slightly too big and heavy to use comfortably in bed, and I'm tall with large hands. When I finally went back to my 11" I was a bit relieved, and enjoyed being able to use my iPad in bed with ease, rather than with slight discomfort and struggle.
The extra weight of the magic keyboard that exists solely to cancel out the weight of the iPad has always bothered me. There has to be a better way.
I use two main devices now: the 11" iPad Pro (sometimes in smart folio, sometimes with magic keyboard) when I want to use it in bed, around the house, or when travelling light, or doing something/going somewhere where a small device makes sense but a laptop doesn't. And a 14" MacBook Pro when I want a bigger screen and need to do more demanding stuff. Both these devices cover all my needs, whereas the 12.9" didn't really fit in to either of those needs.
I'm also struggling to figure out what I want to do with the 12.9". I've been using a 2018 iPad Pro 11" at home for a while now, but only at home due to the pandemic. I'm now attending events and conferences again and I found the iPad Pro 11 to be really great for taking notes, still keeping an eye on emails and messages, and with the cellular option, never having to fumble with flaky wifi logins. All that and the weight/size felt really great in hand as I walked around.
I figured maybe the 12.9" would be even better even though the weight and size are obviously greater. I'm still getting a feel for it right now, but I'm not yet convinced it's going to be a better travel productivity weapon than the 11".
My ideal iPad would be an 11" scaled up to about 12", possibly by reducing the bezels a bit. Something that was just slightly bigger than the 11", with mini LED/OLED, and the same aspect ratio of the 11" (less black bars and height) would be perfect. Chuck the camera in the correct location, enable the smart connector to act as USB-C/thunderbolt passthrough so you could use either the magic keyboard port on the left side, or iPad port on the right side. That would be a pretty meaningful upgrade, and given there are rumours that the 12.9" could be replaced by a 14" iPad, it stands to reason there is at least a small chance the 11" will be sized up to 12".
To me, the 11" tablet should have stayed 4:3 aspect ratio wise. I find a lot of content feels cramped vertically on the screen in keyboard mode. My ideal portrait mode tablet would be the original 9.7" form factor with minimal bezels. In keyboard/landscape, the keyboard size necessitates a slightly larger screen to accommodate a just reasonable enough sized keyboard, so something around 10 to 11 inches in 4:3 would be my ideal.