The Nook HD has a similar form factor to the nook STR - doesn't feel like it will slip out of my hand and the back is shaped similarly. The screen is MUCH nicer than the tablet, IMHO. It feels like there's a zagg coating or screen protector in place to the touch, and the clarity of text is really good. I went with the HD instead of the HD+ because of the size, and because I have no intention to watch videos or do other things requiring a bigger screen.
The clarity is better than my iPad mini, but the reading app is all nook.