I have none of these devices, but was impressed by the information in
this (long) video evaluating scientific PDFs on a 6.8" Kobo Aura H2O (with koreader installed) and an 8" Pocketbook Inkpad. That person picked the 6.8" reader.
P.S. I also feel the same about the Kindle DX not being so great for PDFs, even when using librerator.