Deja Vu.. I've debated this before on friendster.

There were many good arguements.. if only their bulletins were saved back to april or so.. :(

Depends on the venue.

On land the Ninja reigns supreme, but in the water the Pirate is a god!

Well, we know ninjas are unstoppably limber and their senses are sharp. I am not talking about the walk-on-fire-out-in-the-remote-wilderness ninja. Those are mythical creatures. I am talking about the urban dwelling, roof climbing, shadow kissing ninja.
And I refuse to consider the hypothetical scenario if we are dealing with those swashbuckling deckscrubbers with a poor diet. They wouldn't stand a chance against the pantherial chop of cityninjas. But, there are those D.I.Y. pirates that haul ass on their roadbikes with striped socks and a dairy basket strapped to their rack, probably with dumpstered food that they got in the middle of the night against someones wishes. Those "land pirates'" can climb a mean roof and all you need to set them off on a fiery rampage that most definitely ends in property damage, is a few chugs of whiskey. They can disarm you with that "unwashed" smell (I love that smell).
Also, I hear that short females make excellent ninjas, and femme pirates also are as fiesty as the "dudes".
So, to annoyingly refine the question even furthur, what determines the winner? Is it measured in who gets hauled off by the cops or who stops breathing? I think that if the police roll;ed up on a ninja and pirate fighting, they would either both kick his ass together, or flee the scene.