I have seen and used both and I would say go with the tablet. Although the graphics tablets are nice, they don't perform as well. They have a limited surface area that cannot keep accuracy and still enable you to go from one side to the other. With a tablet pc, you can go directly on the screen not to mention that you would have a better grip and usability, making it faster and more accurate.

In your case however, you would want the graphic tablet, because for that small loss of performance and usability, you gain a lot of money back that would normally be used on an extra computer and the expensive touch-screen, and you can use that money on more necessary things for your career. You also have less to install, both on a new computer and installing antivirus, GIMP and other programs, and that you will not have to install your favorite OS and compatibility and such.

All in all, though the tablet PC would be the best overall choice, for a semi-pro, the graphic tablet would be the better choice for someone who hasn't gone full professional.

If I were you, I would definitely go with the graphics tablet. A tablet PC is a lot of money to spend, and let's face it, they aren't as useful as a full-blown PC If it turns out you don't need the graphics tablet's drawing features anymore, you will still have a solid PC that you can use efficiently for other things.

The biggest drawbacks of a tablet PC include a lack of sensitivity and accuracy. The weight of your stroke is transfered when you use anything but the low end tablets. Also, tablet PC's are not (yet) as accurate as stand-alone computers; they're meant for button touches and sketches - not accurate drawing.

So for example, when you're drawing simple lines or filling in colour, you'll need to manually adjust the density / weight of your stroke when using a tablet PC.

The Wacom Cintiq 21UX ( http://www.wacom.com/cintiq/cintiq-21ux.php) is currently the highest and tablet and also what many consider to be the best of both worlds. You get a real time image along with every single tablet feature available.

http://www.wacom.com/cintiq/images/cintiq-21ux-intro.jpg

The lower price ($999) WX version of this Wacom tablet is very similar. http://www.wacom.com/cintiq/cintiq-12wx.php

http://www.wacom.com/cintiq/images/cintiq-12wx-intro.jpg

I would only recommend that you purchase a tablet PC over a pen input tablet if you do most of your work on location. A tablet PC is smaller and much easier to transport. You'll loose accuracy and of course the density input of traditional tablets but you'll be able to work without lugging around a tablet.

One major consideration that I would take into account is the upgrade path. Of the illustrators that I know, few of them upgrade their tablets frequently, if at all. Their computers (mac of PC) on the other hand is upgraded every other year if not more often. Speed is money in many forms of commercial art and a fast computer is key. So, while you may spend $1000 or more on a tablet, you'll save money in the long run by purchasing a higher end computer without the tablet functionality.