What kind of knives do you have? Every sharpener can handle straight-edged blades, but only a few can sharpen serrated (scalloped-edge) knives. However, these knives degrade more slowly than straight-edge ones, so you may not need this feature if your serrated knife doesn't see heavy use. Also, not every sharpener can sharpen sport knives, scissors or Asian-style knives with a narrower edge than a standard kitchen knife. If you want to sharpen these types of blades, look for a sharpener that's designed to handle them.
How patient are you? Electric sharpeners generally get the job done faster than manual ones. They also tend to be easy to use, while some manual sharpeners take a little time to master. For instance, some reviewers rave that water stones deliver beautifully polished edges, but using them takes a steady hand and plenty of practice.
How much space can you spare? Most manual sharpeners are small enough to fit in a kitchen drawer. Electric sharpeners, by contrast, take up space on your kitchen counter or in your cabinets. They're usually about as long as a toaster and somewhat lower in profile.
What's your budget? Manual sharpeners tend to be cheaper than electric sharpeners. Manual sharpeners also cost less to run and, because they don't have any moving parts to go wrong, tend to last longer.
Does noise bother you? Some electric sharpeners can be noisy. On the other hand, the whir of the motor masks the scraping sounds of the blade against the stone. Some people may find that scraping sound -- which is front and center for manual sharpeners -- to be more annoying.
Are you left-handed? Most manual sharpeners can be used either right- or left-handed, as can most countertop electric sharpeners. One belt sharpener we evaluated, however, is made for right-handers only.
How much do you value your knives? Many electric sharpeners, and some manual ones, can over-grind knives and reduce the life of the blade. Warping and chipping can also be a problem. If you have expensive knives that you want to protect, look for a sharpener that doesn't generate a lot of heat and is gentle on the metal.