I got the MK to try with my 11" 2018 pro to see if i like it and maybe go to the 12.9, I don't, not that there is anything wrong with the keyboard it's excellent, i just prefer to use the iPad as a iPad with folio case only, don't like it with no case. I Have a iMac if need a keyboard.
Will be keeping my 11" till it dies, no point trading for the new 11" i can see, i ould be wrong though.
I like the aspect ratio better on the 11", the 12.9 seems to square to me and don't think you get that much bigger screen for video as the black bars are bigger top and bottom, could be wrong there too lol.