To be honest, i didnt know there was a difference
Pepper Spray is a suspension of oleoresin capsicum (OC), a chemical that comes from natural peppers that makes them taste hot. Pepper spray itself is 100s of time hotter than any natural pepper because it is refined and concentrated.
"Mace" is actually two different things. Back in the 80s and 90s Mace was a spray containing Phenacyl chloride (CN Tear Gas). This type of spray is no longer made because Phenacyl chloride is actually kind of dangerous and was found to not be as effective as pepper spray. "Mace" these days is a company brand name that makes a variety of self-defense products, including pepper spray. So you can actually buy Mace brand pepper spray. This is probably why people mix these two things up so often.
Mace and Pepper Spray are considered the same things in the legal system, and neither is illegal in the USA. Mace (Phenacyl chloride) isn't made anymore, but it's not illegal. Pepper spray is legal in all 50 states, though a few states require permits or limit the concentration. Chemical defense spray are only illegal in other countries like the UK and other fascist nations where they don't allow self-defense from assault.