Well, the myth that macs are better at graphics is just that, a myth; it started a long time ago when Thomas Knoll wrote a little program called Display in his mac. Well that little piece of software is one of the most widely used programs in the creative field, but most people know it by the name of Photoshop, presented by Adobe as a commercial app in the mac first. The same thing happended with other indusrty standards as illustrator, inDesign, etc and the myth was born, from the lips of people in the printing industry.
You're in a different industry, and today is not about the tools you use but how you use them. Windows or OS X, both have flaws, one company admits more their problems, and fix them, the other won't admit there's anything wrong with what they make and sometimes releases fixes very quietly. it's all down to the software you use the most and how you use it.
Maya and Max both have strong points, with better support of polygon modeling in Max and Nurbs in Maya. Pretty much both can do the same stuff but the customization of the GUI on Maya and other parts has made it the standard in hollywood.
If you're modeling, you need a single CPU in both apps and a good video card since both are really single threaded for this purpose. For rendering and other functions extra CPU's are necessary in both programs since these parts are multi threaded. RAM (both system and video) allows you to handle bigger amounts of polys.
So if your main soft is Max go for a windows machine, if not either will do the job, but often down the road you will start upgrading parts so usually this is done faster, cheaper, better and with more options on a windows rig.
Take a look at this article on a proper website for people in DCC, not that dreadful site creativecow, it doesn't include tests on a mac, but it does it in one of the main workstations used by DCC pros:
http://www.cgchannel.com/news/viewfeature.jsp?newsid=9305