To make this decision, you must consider three things:
The hand-held scanner is best for scanning small images. It produces a reasonable image at an acceptable price. These usually will only scan monochrome.
The flatbed scanner is best for scanning full pages. It usually produces a better image than the hand-held scanner, but at a higher price. These scanners usually will scan monochrome or color.
Need to do a lot of page scanning? Most flat-bed scanners have an optional document feeder. This device goes on top of your scanner and allows you to place pages in the feeder. The pages are automatically fed through your scanner with no user intervention.
Other things to consider:
Resolution: Measured in dot per inch or dpi. The higher this number, the better the quality of image the scanner can produce. 600 dpi is a good place to start.
Monochrome or Color: If you just plan to scan text, a monochrome scanner is sufficient. If you plan to scan color images, a scanner with color capabilities is a must.
Maximum Document Size: If you plan to scan legal-sized documents, make sure to buy a flat-bed scanner that will handle 8.5 x 14 documents. Most flat-bed scanners will handle this size document. But some won't. To be on the safe side, check.
Scanning speed: Some flat-bed scanners are slower than others. If you plan to do a lot of scanning, buy one with a fast scanning speed. This will save you time in the long run.