If you're comparing the Wacom Cintiq with a 'carpc' quality touchscreen, that's like comparing a Power Wheels to a Porche.
While the accuracy of the touchscreen depends on what type of touch element it is, it's nowhere near as good as the Wacom, as the Wacom has several levels of touch sensitivity as well as a higher resolution to the touch panel itself.
While my touchpanel is fairly accurate, I don't think it's accurate enough for reliable photoshopping, and the drift in the panel does get fairly significant. If you want to get one, give it a shot, but it may be more frustrating than you want. But...i've never tried photoshopping in the car, so i'm not one to talk.
It -may- be good enough for what you want, depending on the screen you're looking at and if you're going for a 'better than nothing' approach, or don't like playing the hand-eye coordination game with a standard tablet.