Size and Shape
Consider what you would like to cook in your pressure cooker and choose an appropriate shape. Good pressure cookers are the right shape and capacity for the dishes you want to cook in them. Some pressure cookers are big and oval-shaped, making it possible to cook whole chickens or turkeys. Some are smaller, or are shaped such that they're best for stews or soups. Some are shaped and sized to work well for canning.

Material
Pressure cookers generally come in choices of aluminium and stainless steel. Aluminium cookers are lighter and cheaper, but tend to look less attractive, and there are some concerns over food safety issues related to cooking in aluminium pots. Stainless steel is more costly and heavier, but does not tarnish and is chemically inert.

Pressure rating
The level of pressure reached by your pressure cooker determines the temperature it can reach, and therefore determines the cooking time. The higher the pressure rating, the faster your food will cook. If time is an issue, choose a pressure cooker that reaches 115 or greater. There should be a safety pressure release valve on the lid of the pressure cooker.

Seal
In order to reach high pressures, it is necessary to have a good seal. Different models seal in different ways. Some have a lid that can be sealed by means of a rotating clamp system; others have a simple screw on top. Older style pressure cookers often have a lid that actually goes inside the cooker itself and is forced upwards to form a seal on the inside rim of the pot. Most involve a rubber O ring which must be replaced when it becomes worn.

Valve type - adjustable or fixed
Some valves are fixed, and cannot be adjusted. Pressure cookers with these valves will reach the rated pressure and stay at it if there is enough heat. There are also adjustable valves, which enable the cooker to operate at different pressures depending on what is being cooked. Adjustable valves often come in the form of weights that can be added or removed from the valve stopcock, but high quality models may have more sophisticated systems of pressure adjustmen.

Safety
Pressure cookers with electronic sensors tend to be safer. It's sometimes possible to force open a jiggle-top pressure cooker during the cool-down phase, while the pressure inside the cooker is still greater than that of the surrounding air. This can make for some real messes and some minor burns. Pressure cookers with electronic pressure controls and safety locks tend not to have this problem.

Handling/Cleaning
The pressure cooker should have a handle that resists heat. The interior of the cooker should be easy to clean with hot water and soap. Some pressure cookers have decorative coatings that are harder to clean than others. It's often a good idea to get a cooker that wipes clean easily. No matter how careful you are, you'll end up spilling food on the outside of the cooker.

Replacement Parts
Look to see if replacement parts, such as the rubber seal, are available for the model you are considering. You don't want to invest in something and then later not be able to find parts for it.

Customer Service
Customer service is important when choosing any appliances. Look to see if your cooker has a customer service number to call with questions. A web site is an added bonus.