You left out battery life and weight, I think it would have won those. Anyway like Kyuss72 said it's usability.
I have a Motion Computing LE1600 which is similar to that HP tablet. I tried an iPad last year. Wanted to draw on it, do art, use it like a Wacom tablet. Didn't work, I wanted a stylus. Returned the ipad. Researched and found about the Motion LE's. Fab piece of kit, Wacom pen etc, use it with the proper Sketch Book Pro and Windows 7. I love it and only £100 off eBay.
But...... tried it on the sofa for web browsing etc. and while okay it's heavy and needs to be plugged into mains.
Just got an original iPad again, refurb from Apple £289. This is a fab piece of kit also. Using it for what it is intended for. I can't see me browsing the internet on anything else now! Use the Motion for pen drawing! Horses for courses. Their is a new Acer tablet out this year. Windows 7, multiple touch and Wacom pen. Great, but heavy and 3 hour battery life.
Apple has reinvigorated the tablet market. There were tablet PC 's 10 years ago but they didn't take off.