The scanners can track the pen's location because the pens have tiny bar codes on them that the scanners can see. If you don't have the special pen, the device won't work. The resistive membrane boards, on the other hand, are more susceptible to damage because of their soft writing surface and often can't automatically detect what color marker you're using. From a usability standpoint, all except the laser scanning boards require you to press the marker harder against the board than you might be accustomed to. But while the laser scanning boards don't share this problem, they also tend to be the most expensive.
Portability is another consideration.
The sonic whiteboards shine in this area because they attach to an existing whiteboard. The electronics for these devices will easily fit into your computer bag allowing you to take your digital whiteboard on the road or quickly move it from room to room. All of these products connect to your PC, and the software on the PC determines how easy the board is to operate. The software from one manufacturer may have special features that you want such as thumbnail views of captured whiteboard images, handwriting recognition for turning your notes into editable text, or the ability to export the images into standard graphics file formats. Lastly, be sure to check the warranty of the board's electronics and writing surface before purchasing.