Pepper spray is a generic
Pepper spray is a generic term for all self-defense sprays that contain oleoresin capsicum, a carrier, and a propellant. Oleoresin capsicum is the active ingredient in pepper spray, harvested or derived from cayenne peppers or other peppers. Its effects are physically incapacitating rather than painfully irritating; this makes it ideal for self-defense sprays that are intended for use on assailants who are immune to pain (psychotics, those under the influence of drugs or alcohol, or just "tough guys").
Tear gas, on the other hand, is commonly made of one of two chemical compounds: CS (orthochlorobenzalmalononitrile) or CN (choroacetophenone). These compounds, when sprayed into the face of an assailant, work to irritate eyes, skin, and respiratory system. They work on the pain principle, and so are not effective against those who are impervious to pain.
Mace is a brand name that used to offer only tear gas. Now, Mace offers several different types of self-defense spray, including:
Mace Triple Action is pepper spray, tear gas, and Ultra-Violet dye. The pepper spray physically incapacitates the assailant by forcing his eyes shut and tightening his bronchial tubes, making it impossible for him to see and very difficult for him to breathe. The tear gas irritates the bronchial tubes, causing him to cough. The UV dye brands the assailant with a purple mess, making him easily identifiable.
Pepper Mace uses a 10-percent oleoresin capsicum solution and UV dye. The OC solution is a particularly strong one; it doesn't kick in as quickly as a smaller concentration like 1-5 percent, but the effects are longer, sometimes up to 45 minutes.
Pepper foam is the same as pepper Mace, but it sprays as foam rather than a liquid.
So as you can see, it is not a question of mace vs. pepper spray, since Mace is simply a brand name. The question is really one of Tear Gas vs. Pepper Spray. The winner? Pepper spray, hands down.