650 v 600 keyboard
I've owned both.
I think the actual typing speed is about the same. The keyboard experience is comparable. Personally I find this okay.
Make annotations is much faster on the 650. The delay opening the annotation interface, the erasing, the marking, etc are all much faster on the 650 and perfectly usable (unlike on the 600).
Originally I didn't have an SD card, and I think I started to notice a slow-down with annotations, but nor sure. Now I store all books on the SD card and there is no slow down where many annotations are used. I did the same on the 600 and it worked there too. Keep books on external card. Leave base RAM free.