Plain simple, pound for pound an AMD based PC will give best value (speed wise) for money. However OS-X is a more stable and nicer working environment the windows GUI is ok once you're used to it, it's stability that lets windows down. (Office however is very resource hungry)

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Toolset: Blender, GIMP, Indigo Render, LuxRender, TopMod, Knotplot, Ivy Gen, Plant Studio.