I think Assassin is a better name for Ninja as well. For Samurai, I think any of the following might work due to the playstyle of the class: Duelist (As Delf said), Mercenary (As forzaken said), Swordmaster, Brawler, Berserker, Spartan.
The platsyle of the class involves powerful DoTs and speed blitzes, not a survival class but one that focuses on taking down as many of the enemy as you can before you bite it yourself. Berserker or Spartan might be a good name. I'm not sure at the class' usefulness CC versus duelling, so if a Sammy is better at 1v1, Duelist might be a good name. If it's better at CC, Duelist might not be as good a name.