What kind of power source do you want? Electric toothbrushes are usually rechargeable, but some operate on standard AA batteries. Battery-operated models are usually less expensive and more portable, but rechargeable models are typically better reviewed.
How often do you travel? If you travel a lot, a travel case may be necessary. Uncovered toothbrushes can be a target for germs and may be inadvertently turned on in transit. Battery longevity and the portability of a brush's chargers are also important considerations for jet-setters.
Do you need a compact brush? Children, in particular, require small brush heads, but some adults also have trouble maneuvering large brushes inside their mouths. If this is the case, look for a compact brush.
Which brush settings are a must-have? Many brushes have several brushing modes. Decide before shopping whether you require a setting for sensitive teeth or for whitening.
Does design matter? If your toothbrush's charging stand will be sitting out on your bathroom counter, looks probably matter. Decide how much a compact or stylish stand is worth to you.
The least expensive electric toothbrushes (up-front, at least) are battery-operated brushes. However, these can have hidden costs, including replacement batteries -- and for brushes that don't have replaceable heads, replacing the whole brush every three months. When you upgrade to a rechargeable brush, you typically get added quality and durability in addition to eliminating battery replacement costs, though the up-front costs can be tenfold or more. Additionally, replacement brush heads can be very expensive -- $10 or more for some models. In some cases there are generic brush heads compatible with brand name brushes, but not always.