Yes there is a difference between mace and pepper spray.
Mace is more like a tear gas spray while pepper spray is more an irritant using pepper and other chemicals to cause irritation to the persons eyes and face to stop the attack.
Civilians can usually buy pepper spray while regular mace is mostly used by and sold to police to use when dealing with suspects that won't comply.
Mace is usually illegal for civilians to have in their possession while pepper spray is usually legal in most states as long as you use it for self defense.
You can't just go around spraying people with pepper spray but you can use the pepper spray when actually needed to stop an attack.