I would have to disagree with you on several points:

1. the pain threshold counts for much less than you'd expect. Especially if we are talking about the halan, or black eggs, which not only cause pain but legitimately cause blindness. pain is only a secondary factor. if you can't see on the battlefield, you're dead, and the ninja knew it.

2. the spartan's close-combat training may have been great, but the ninja employed long-range weapons that could drop the spartan before he ever had a chance to use that training. A spartan's shield and armour count for nothing when he is against an enemy using poisoned weapons AND stealth to get the best firing angle.

3. ninjutsu no only has a grappling element, but relies on it heavily. The Naki family in particular was exceptionally proficient in breaking bones, even of armoured opponents, which takes the encumbering bulk of the spartan's armour and makes it nothing more than a bronze deathtrap.

I've studied ninjutsu, their techniques, history, and mindset extensively. I have great appreciation for the spartans and their skill but the simple fact is that if 300 had ten ninja instead of the persian horde, Leonidas and his group would have gone to the pass and found no-one, then returned to find all of their women and children fled or slain, and provided they did not themselves submit to the same fate as their families, the spartans would then have been picked off by attacks they'd never seen coming. The ninja were masters of taking their enemies out of their comfort zones, and so would have totally baffled the spartans until it was too late. Victory: ninja

I have to shine some fact on your second and third arguments:

ninjas didn't have "bushido", the constraining code of honour that the samurai had. they only learned bushido so they could use it against their ancient enemy. The only loyalty or honour a ninja has is to his family and clan. when he's fighting all bets are off.

secondly, ninjas do nothing but find "new and innovative" ways of destroying their opponent (read: they played dirty) there's a move in ninjutsu known as "monkey steals the peach" that ends with the enemy bleeding profusely from the place his manhood used to be. now that's below that belt :P

Argh what is it with you people and thinking ninjas do that in combat? ninjas were agile so the could infiltrate their targets and get into areas they wouldn't be spotted. when it comes to combat ninjas strike so quickly half their adversaries never got the chance to draw their weapon. and I am proof that ninjas are not dead.

1. the first historically recorded ninja was Otomo-no-saigin, who singlehandedly ended a 10-year war for his employer in six months. the fifteenth to period you're claiming they originated from is merely the period in which they were most notorious, while Otomo's time was in the SIXTH CENTURY

Second: brutality is nothing when ninjas were known to decimate entire villages without ever setting foot in its walls.

Third: no organic creature is faster than lighting, but ninjas were still damned fast, and definitely fast enough to decapitate a pirate before he could draw his pistol. as to super powers? I'll let Arthur C Clark say it for me: Any sufficiently advanced technology is indistinguishable from magic.

Last: ninjas are not myth. I am one, as is my master, and his master before him. my training lineage goes all the way back to the 8th century. and we don't attack because we don't have to , though word's going around about finally doing something about north Korea.