Do you plan to carry the pepper spray on your person? If so, consideration should be taken regarding the size of the unit you choose. Keychain pepper sprays are easily carried and accessible since you can have your keys in your hand to and from your vehicle or residence.
If you are planning on carrying a unit while you exercise (such as walking, jogging, bicycling) you might want to consider a pepper spray with a belt clip. These are easily accessible if surprised by an attacker while you are out and about.
For home or auto use, there are several larger pepper spray models that contain more chemical and can be available to you whether you are on a long road trip or your vehicle breaks down.
If you would rather carry pepper spray in a covert canister, the pepper pen or pepper pager are perfect disguises for hiding the defense spray in plain sight.