I run both Mac OSX and Vista platforms, so let me tell you what I have noticed in my own personal observations:

The OSX version of WoW is not as efficient as the Windows side. The same processor and system with both systems will not run as well with the same graphics settings on OSX as on Vista. I personally believe that the performance issue has more to do with graphics code optimization for DirectX as opposed to OpenGL.

So, if you are looking to get the most "bang for the buck" I would advise that you use Windows XP/Vista.