They are different species. They have different dorsal ray counts, different clod patterns (although very similar a lot of times), and different geographical locations. The easiest way for me to tell them apart is to look at the dorsal fins. If the first dorsal ray is rather short and makes the dorsal fin look more or less like a uniform hump, you are probably dealing with percula. Conversely, if the first ray is rather tall and makes the dorsal look more like a sail, then it is most likely occellaris.