Color
White is predominant, but most manufacturers offer a variety of hues. Before buying a color, make sure you're ready to live with that color. Also consider resale value. Some people might not like your choice of color.

Style
Do you want to convey a look that's contemporary, traditional, casual or formal? Styles usually don't affect the operation of the toilet so it won't matter what style that you pick.

Gravity or Pressure
Gravity-fed toilets work the old-fashioned (and most reliable) way. Upon flushing, water goes from the tank into the bowl (by means of reliable gravity). When enough water builds up, the weight forces everything through the S-shaped trap way, at which point a siphoning action takes over and finishes the job.
Pressure-assist tanks are completely sealed and rest inside the toilet housing. When the tanks fill with water, the trapped air gets compressed. The pressure-assist results in a more efficient flush. A more efficient flush means that the water surface in the bowl can be larger. And a larger water surface means less frequent cleaning. The potential drawbacks are that pressure-assisted tanks are louder and sometimes tougher to repair than conventional gravity models.

One-piece or two-piece
Most toilets have separate tanks and bowls, making them two-piece, but many higher-priced toilets are one-piece, and generally more stylish. This is a style preference. One piece tends to cost significantly more.

Roughing
The distance from the finished wall to the center line (point where the bolt connects to the floor) for most toilets is 12 inches. With older installations (or mistakes made during construction), this distance may vary from 10 to 14 inches. Also, some foreign toilets might not have 12" so be careful.

With a seat or without a seat
Most two-piece toilets do not come with a seat. Some one-piece toilets do. If you buy a toilet without a seat, be careful that the toilet that you purchase does not need a unique shaped design. If it does it could present a problem in the future when you need a new seat.

Round bowl or elongated bowl
Most bowls in residential installations are round; in commercial installations they are usually elongated. Many bowls are made in elongated versions as well; you just have to ask for them. With an elongated bowl, you get an additional two inches in front. That means a larger target area and less drip on the bowl.