Really isn't serving the same purpose as an alignment system. Evidence: you could have both present in the same game without them clashing or stepping on each others' toes.
An alignment system as D&D uses it - even restricted to "behavior reveals alignment" style "descriptive alignment" - ties your personal desires and methods to cosmic forces. This Zodiac concept would tie something more akin to character race to cosmic forces. (A player would choose the sign under which his PC was born the same way he chooses the race he was born as.)