ScannerGalaxy has a guide for picking scanners.
The follwoing was taken directly from ScannerGalaxy.com.
Choosing the Right Scanner
When choosing a scanner, there are several key attributes that you
should take into account.
1. Color Depth: The number of bits captured per pixel, which is related to the number of possible colors. Higher color depth equals
better image quality. Typical color depths are 24, 32, 36, 42 and
48-bit. If you plan on doing document scans, then you may not even
need color. However, for those that wish to scan photographs or
projects requiring great detail, then look for a scanner with a color
depth closer to 48-bit.
Optical Resolution: A measure of how well a scanner can capture an image. It is the actual number of pixels that the scanner provides
when scanning an image. The higher the optical resolution, the higher
the quality of the image captured. Measured in dots per inch (dpi).
Not to be confused with interpolated resolution. 300 dpi will do well
for most office applications. However, look for at least 600 dpi when
photos are involved.
Interpolated resolution: The ability of the scanner software to "fill-in" spaces between scanned dots. This is really only a factor if
you plan on enlarging images. The higher the interpolated resolution,
the smoother your enlarged images will look. 9600 x 9600 dpi is a
typical interpolated resolution offered today.
Flatbed vs. Sheetfed: Flatbed scanners offer a flat, glass surface. The image to be scanned is placed on the glass surface and the lid is
shut. This is typically used for environments where very limited
scanning takes place or for "irregular" objects (i.e. a book or hard
cased object). Sheetfed scanners are useful for environments that
require frequent volume scanning. They allow you to scan large
projects automatically. Simply place the sheets that need to be
scanned in the automatic document feeder. The sheetfed scanner will
automatically process the sheets and scan them automatically. Scanners
offer various sizes of automatic document feeders. Larger capacity
feeders allow for larger jobs to be completed with fewer interventions
from the user. Some scanners offer both flatbed and sheetfed
capabilities. This allows for more flexibility in a single unit.
Simplex or Duplex: Simplex scanners have the ability to scan one side of a document. This is useful when a majority of your items that
need to be scanned are single-sided. Duplex scanners allow for the
scanning of both sides of a document in a single pass. A majority of
duplex scanners have a feeder with a dual sided lens that reads both
sides of a document as it passes through. Duplex scanners can be set
to scan as simplex or duplex, depending on the job the user wishes to
Scan Speed: Scan speed is typically rated in pages per minute (ppm) for simplex scanning and images per minute (ipm) for duplex scanning.
Pages per minute (ppm) measures the amount of pages scanned in a given
minute. Images per minute (ipm) measures the amount of actual images
(front and back) scanned in a minute. The higher the scan speed, the
more scans a user can do in a set amount of time. Look for higher scan
speeds if you plan on doing frequent or high-volume scanning. Scan
speed decreases as you increase the scan resolution.