Historically speaking;
Pirates were often drunk, malnourished and easily bought out by opposing armies,lords or gangs.
Ninjas were very disciplined in meditation, eating well, trained like Olympic athletes and were loyal to the death.
Again, i gotta go with Ninjas
Vikings perhaps?
I think I would go for pirates if I had to choose between pirates and ninjas though.
Ick, vikings. SO old-fashioned. I disagree though, not only are ninjas intelligent as well as strong, they're also agile and tactful. They would most DEFINETLY beat vikings.