1) Pirates have boats... And the ladies love a man with a big... Vessel. Ninjas? Ninja's have to walk everywhere. Walking is for losers.
2) Pirates have guns. And Cannons. (Tim Allen-circa-Home-Improvement-1994-esq Grunt) More power!! Ninjas have... Throwing stars, which are kind of cool, except when you throw one and miss and it gets stuck in a tree. Have you ever tried to get a ninja star out of a tree?
3) Pirates get Wenchy lovin'. Ninjas are devoted to a life of celibacy to study their craft. Lame.
4) Pirates have rum. Rum is good. Rum goes with everything! Juice, Coke, Pepsi, fresh fruit... Sake goes with the hole it's eating in your stomach lining.
5) A Pirate is still a pirate, even if he's missing an eye, hand, or leg. A Ninja is no longer a ninja if he's missing any of those things. He then becomes the owner of a local Panda Express Franchise.
6) Pirates have cool companions like Parrots, Giant Squid, and undead monkeys. Ninjas have... No, seriously, what do ninjas have?
7) Pirates steal stuff. Nuff said.
8) Pirates have their own ride at Disney world.
9) Water covers 3/4 of the Earth's surface. Ninja's are ill equipped for aquatic combat, as they have no boats. Pirates win.
10) Pirates have gold. You can buy things with gold. I like buying things.
You may disagree, and argue that pirates are smelly, drunken, dishonest, disorganized thieves, but let me ask you something...
... Have you ever downloaded a song or a movie that you didn't pay for?
PIRATE!