As far as I know most Tablet PCs don't come with dedicated graphics cards so you might not be able to push as many polygons, or use any fancy real time shaders. Looks like some of the newer models have SXGA+ 1400x1050 screen resolutions, the older ones mostly had 1024 x 768 which would be rather small for 3d apps.
Modeling with a Tablet PC's pen would be no different from using a wacom tablet to model with, except you would be pressing on the screen rather than the wacom of course. You might also need to pick up a duo switch wacom
pen or a Cintiq 15x or Cintiq 18sx pen so that you could have 3 mouse buttons, rather than only 2 buttons that the standard tablet pc stylus comes with. Note that the newer Cintiq pens are not compatatble with tablet pcs.
One other consideration is that if you buy a convertable tablet pc rather than a slate version then you would also need a separate keyboard or other input device to use for your hotkeys, as the pen alone isn't enough for most 3D apps.
In the end unless portability was your main concern I'd say spend your money on a decent desktop computer such as per's new machine
and buy a Cintiq instead.