As a couple of people mentioned above, the heat of the pepper spray, measured in SHUs, determines the effectiveness, including how fast it takes effect. The Fox spray is the only 5 million SHU spray I have ever seen. Most on the market are in the 1-2 million SHU range.
The percentage of pepper determines how long the effect lasts. In general, a 10% spray will be less effective than the Fox spray, but the burning and swelling will last much longer. So if you are using a low heat spray that just aggrevates the dog, the high percentage means that the dog will stay aggrevated longer.