Well, since I don't know either class, I'm, not biased, but I also don't know them at all. I'll share my opinion after reading spirit shaman.
Not done reading yet, but seems shaman is more of a guide; spirit shaman is more of a mediator. Hmm spirit shaman casts on wisdom, but DCs are on charisma. Peculiar...They seem similar, but different. They are like cleric and druid or fighter and barbarian; maybe even wizard and sorcerer. They are however different, in both crunch and flavor. I don't understand them well enough to speak about them, so hopefully someone else has an answer.