Wacom penabled technology is a must for the artist. Be sure the tablet you purchase uses Wacom penabled technology (I believe Wacom lists the systems using it on their website). You'll probably need to download the proper drivers from the Wacom website as they don't usually come installed.
I have found that drawing on my wife's Motion is the same experience as on my HP. The Cintiq has 1012 levels of pressure (at least on the 21" model), tablets were at 256, I think newer ones are now 512. Pen settings make a big difference, be sure to have the proper drivers installed and ensure the pen settings are consistent if comparing.
Friends with both the Cintiq and Tablet PC have said the pressure sensitivity difference is not such a big deal - the larger screen size of the Cintiq is what makes the difference. If looking at the 12" Cintiq I would likely always pick the tablet as you get an entire computer for the price of the Cintiq monitor.
I would like to get a 21" CIntiq for my home set up, but really enjoy the portability of the Tablet PC. Check out
www.cartoonmonkey.com for some more Tablet PC artist info.
Safari is another Slate type Tablet PC brand you might want to investigate.