With a flatbed, having to switch pages and wait for the scanner to warm up can be a chore. Inserting a stack of documents in a sheetfed scanner is much more convenient and takes less time. What flatbed scanners lack in their portability, however, they more than make up for with the quality of the scans they register. On average, flatbeds are much better at reproducing color documents than sheetfed scanners. Sheetfed scanners also sometimes grab a photograph and scan it in at an angle, resulting in a crooked image and a wasted scan.
Flatbeds also have the upper hand when it comes to the type of material scanable.
Because of their design, flatbed scanners can reproduce an image from a book or magazine as easily as a photocopier. Once you've decided on the design of the scanner you want, there are a couple of other factors to consider. The higher the number, the better scanning results you'll get. SCSI scanners offer faster performance, but they often cost more and are far more difficult to install. If you're not that familiar with opening up your PC and installing new hardware yourself, go for the parallel version.