NINJAS. They have the stealth. You have no way to counter because you don't see them before they KILL YOU. Pirates are like HO HO HO WHERE'S THE BOOZE MATEY and then you crack them over the head with the empty bottles littering your ship.
Pirates. Ninjas think they're all kinds of badass with their sneaking around pretending they're like some kind of force of nature, whereas pirates are just badass. Also, pirates can do pretty much whatever they want. Ninjas all serve people. Laaaame.
Ninjas are the sh** there sneaky, have cool weapons, and have supernatural martial arts skills. Pirates shoot people with cannons and guns have no martial arts training and rape and pillage people. there not bad asses there just assholes.