There are six considerations when selecting OC - Oleoresin Capsicum (aka Pepper Spray). Mace is a brand name like Kleenex.
1. SHU's - the higher the number of SHU's the hotter the forumla - Fox will have the highest at 5.2 million - nearest on the market is 500,000 to maybe a million
2. Percentage - higher percentage only means longer detox time not strength or integrity of the forumla (2% Fox is considerably hotter than Brand X with 5% because of the SHU not the %). The higher the % the longer it will take to stop burning. For Self Defense you only need enough time to elude, escape, or gain some distance.
3. Flammable or non-Flammable - check the MSDS or label - some brands use a propellant that can/will catch fire. Generally not an issue with private citizens who don't plan on taz'n someone!
4. Size of container - Trust me on this one...the less training you have (vis a' vis no training probably) the faster you'll go thru the juice! Most inexperience people dump the entire can in the open second of attack and then are in trouble later (because you've ran out of OC)
5. Type of delivery - foam, fog, cone, or stream? There are tradeoffs on all four
6. Storage - most people compromise their OC because they store it incorrectly. It must be kept in reasonable temps and it must be stored when not in use upside down - notice LEO generally carry their OC upside down - to keep the O ring lubed and prevent it from drying and cracking. Also OC must be shaked to mix the compointents periodically - once to twice per week for at least 8 seconds - otherwise the Capsicum seperates and settles to the bottom and all you're doing is spraying with the propellant.
7. (Bonus point) - Okay, I said six points here is a bonus! Get some Professional Training! You can be held liable for using this stuff if done in approprately and ignorance doesn't play well in court! Also if you carry this stuff then carry Suedicon decontaimination wipes - trust me again on this. If you hose someone then there is a 80% chance you'll get primary or secondary contamination from your own spray! Also have a back up plan if the OC fails to stop the attacker - it works about 70-80% of the time in most studies.