I've not used AutoCAD Architecture yet, but i know a designer who swears by the ACA-Green Building Studio link, and says it has the additional capability of exporting constructions for energy analysis that Revit 2009 products don't have.
My understanding of the reason it's not promoted in the GBStudio literature is that in ACA users have the freedom to create 2D models and explode 3D intelligent objects so that they become 2D lines. Since the use of Building Information Modeling (BIM) with GBStudio requires 3D intelligent objects to communicate critical information about the building to the GBStudio web service for energy analysis, the ACA workflow where a model can be designed entirely without such objects, or where a model that does contain such objects can be broken, challenges the foundation of compatibility and interoperability with GBStudio.
That said, if you or your company have a policy that would ensure your ACA models will rely on BIM objects, ACA with GBStudio could be just as good an investment as Revit with GBStudio.
Thanks for your question.
Omari N.F.
Green Building Studio Product Design Engineer