I use a Cintiq 21ux at work, prior to that we had a 17sx at work.
Though the 21ux is supposed to be more sensitive with the stylus, I haven't noticed the difference.
The biggest advantage with the 21ux is the incredible screen size. I would recommend having a video card that supports dual monitors, and gettingt a cheap 17" monitor for all of your tools and pallettes. If you have the extra cash, I would also invest in the wacom tablet accessory for the Cintiq which will allow you to interface with both monitors with one stylus.
Having the entire 17"/21" screen available for work is indispensable.
My recommendation to you is get whichever Cintiq fits your budget, but once you go 21" you'll get spoiled and never want to go back.
"Form follows function." -Louis Sullivan