That's next years model! LOL
Actually, I've seen the prototypes of the bendable OLED screens, and wouldn't be surprised to see that implementation in an iPhone or iPad in the next 3-5 years.
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This is a tough question as there are pros and cons to both.
I have the 12.9, and I will honestly say that it took me at least 6 months of trying different cases, keyboards, and other accessories to find the right ones to make using it comfortable and easy.
Currently I'm using something called the Torch Grip (
https://www.torchgrip.com/). This is the first solution I found which actually makes the 12.9 comfortable to use while holding. Using the device naked makes it slippery and a bit awkward, and using it with a case makes it heavy.
I use this in combination with the Apple Magic Keyboard. It works well at the desk, and the handle of the torchgrip is removable for when you want to travel (it clips off and on with a button).
That said, I'm considering "upgrading" to the new 10.5. I find the 12.9 is just a bit too big to take with me when I go places. Because of its size, I find myself being lazy and often times leaving it at home and just taking my iPhone when it would be better to take the iPad. I think with the smaller size I am more likely to use it more like an iPad and less like a laptop.
So I think that is really what you have to ask yourself.... if your intended use is around the house and you don't plan to take it places often, then with the right accessories it should be good. But if you want it to be more portable, definitely get the 10.5.