How much do you plan to use it? If you need a steamer just for occasional use -- or to use in only one area of the home -- a smaller steamer would work just fine. Even a handheld steamer that only produces 15 minutes of steam per fill may be sufficient.
Do you need it for travel? Travel steamers, also called handheld or compact steamers, are a great way to quickly de-wrinkle suitcase-creased clothing. Be sure the one you choose is small enough and light enough to fit in your suitcase. Also, if you tend to run short on time, be sure you get one that does not leave water spots that may need time to dry.
What feels comfortable? You need to hold the garment steamer in your hand to see how it fits. For handheld garment steamers, make sure it is not too heavy; keep in mind you will be adding water to it.
Do you need an iron? Garment steamers are great for just smoothing wrinkles out of fabric, but they don't give the pressed look that ironing does. Either look for a garment steamer with a crease attachment, or see our report on steam irons.