A longtime customer called asking what features she should look for when purchasing a new toilet. She was a little overwhelmed at the choices she had, so I gave her some of the high points of buying a new toilet.
The first consideration is the color and whether or not she wanted a one-piece or two-piece toilet. White toilets are more popular and so you have more choices and options. One-piece toilets are more expensive and therefore are not as popular as two-piece toilets. If white isn't your color, almond/bone is usually stocked, but other colors are available through special-order.
The next feature is a round or elongated bowl. A round seat is smaller and will fit in a smaller space. The elongated bowl takes up more space but is more comfortable.
The height of the bowl is another consideration. You have a "standard" height, a "comfort" height, and a "chair" height. Toilets with a rim height of 16 ½" or more comply with ADA and offer ease of use for tall people or disabled people.
The flushing system a toilet uses is important, and you have a choice between gravity-fed and pressure assist. Most toilets are gravity-fed, meaning that the toilet uses the natural force of gravity to send water from the tank to the bowl. Pressure assist uses pressure to force the water into the bowl. You get a stronger flush with pressure assist, but the cost to repair these is MUCH higher than a gravity-fed toilet.
To me, the most important feature is the size of the trapway. The trapway is the passage that carries the waste from the bowl to the drain pipe in the floor. The larger the trapway, the better. You can get a toilet with a trapway from 1 7/8" to around 3". The larger the trapway, the fewer clogs you will have. It's also better if the trapway is fully glazed so the passage is smooth. Along with the trapway, is the size of the flush valve. This is the size of the hole in the tank that allows water to flow into the bowl. The larger the flush valve, the stronger the flush will be.
You might also consider the water surface inside the bowl. The larger the water surface, the easier it will be to keep the bowl clean, and it will help to reduce odor.
Yet another consideration is a dual-flush option. These toilets have a flush mechanism that allows for a "half-flush" for liquids, or a "full flush" for solid waste. These can considerably cut down on your water usage. Here is some information on how to replace a toilet.