I don't have first hand experience with the Nook and have obtained the information below from the user guide. Look forward to other people's comments who have first hand experience.
One key difference is that the Nook has touchscreen navigation and keyboard that must be accessed with finger touch and not stylus/pen/adapted pointer touch. I would assume this requires less pressure to select than the Kindle mechanical buttons for navigation/keyboard. It may be more intuitive in that more of your visual and motor points of interaction are the same vs. the Kindle with buttons on the bottom to move through screen on the top for everything but "Home" and page turn.
I am not sure about the size of the Nook touchscreen buttons but assume the size cannot be adjusted. The touchscreen keyboard size for the Nook is also fit within a 3.5x1.5"(approx.) space which would present issues. Considerations for kinesthetic keyboard input need to be made with the Nook (touchscreen) vs. Kindle (mechanical buttons). Page turning options with the Nook include mechanical buttons or finger swipe on the touchscreen but no finger swipe option exist with the Kindle. However, some have reported the finger swipe can be motorically complicated.
What have other people found in terms of motor access for these two devices?