Samurais are pretty good, but they have two distinct disadvantages. First, it will take a lot of time and training to access the Samurai class, whereas the Monk class is available very early in the game. Second, the Samurai skills have a significant cost attached to them - especially considering that katanas like the Masamune and the Chirijiraden are in limited supply - while Monk skills are all 100% free. All in all, I guess it just depends on your style of play.