Why compare it to MS64-bit? Windows XP 64-bit it crap right now with shit driver support. It doesn't even give a performance boost becuase nothing is written in 64-bit for it. It's like comparing a bmw to a ford.
Windows 64-bit not designed for the home user and for CAD? Are microsoft complete idiots? The only 64-bit CPU's are AMD, and AMD's are much worse performance wise in things like CAD, 3d modeling, video encoding. But AMD's do very well in gaming against Intels.
I think tiger wins this one. But in my opinion Linux does because it doens't need an apple to run on.