It's hard to tell. The guy in the demo didn't say that the samurai as a whole are speedier but less defensive than the other factions. He said that his samurai assassin was as such. Which might mean that Vikings and Knights have classes which are equivalent.

I'd personally like to think that the differences are more in the weapons and armor and in special abilities (like the Samurai arrow call) than stat differences built into the classes as a whole.