Ninjas would win due to superior stealth. Firearms are more effective than shuriken, but a shuriken that comes from somewhere you can't see unexpectedly is far more effective than a bullet that can't hit the target because the guy who squeezes the trigger can't see the target.
SUCKMYMUSCLE
Oh boy, now you're a ninja expert as well??
btw, you also misunderstand the use of Shuriken totally. THey were used either as decoy, in close combat held in the hand or with poison to poison someone from a distance.
Shuriken only make superficial wounds when thrown, and if thrown, they are only thrown in a distance of 2-3m. Everything else you read or hear is bullshit.