coming from someone who's worked in print and web design, the main reasons that i prefer working on a mac are really quite simple, and in fact have very little to do with graphics:
--the mac os is less complicated than windows if you know how to use both fluently. anyone who says otherwise doesn't know how to use both fluently (i.e., they know more about windows than the mac os). i'm not saying that they have to like mac better than windows, it's just a case of simple user interface design: i don't have to open 6 different dialogue boxes in the mac os to change one printer setting, let alone try and source a printer driver, install this or that bit of software to drive some peripheral etc.
in short, what takes me half an hour in mac os takes longer in windows, which means the time my client is paying me is better value on the mac, because more of it's going toward actual work rather than just using the os.
--apps on the mac crash far less often than on windows, and the actual os rarely (if ever) crashes. when an app crashes, in most cases, it's possible to recover work. on windows, crashes are a lot more frequent, and are harder to recover from. if the windows os crashes, you can say goodbye to all your unsaved work (and this happened to me at least once a month every time i used windows, more commonly weekly or fortnightly)
in short, again, far less time screwing around with the os, more time doing work
--the mac os has been designed with the specific hardware of the computer in mind, meaning it's more efficient when running programs. this is noticeable in any app, but is really, really noticeable in heavy apps, like, for example photoshop. i.e. 1gb ram in windows and 1gb in mac are not equal, because windows uses more ram to process the actual os than the mac os. i don't know exact figures, but say you're running photoshop with 1gb ram:
mac os might use a quarter to a third, leaving 3 quarters to 2 thirds for photoshop, windows might use a third to a half, (or more, under vista) leaving 2 thirds to half (or less) for photoshop. meaning photoshop will be slower with the same specs under windows.
in short, windows uses more of your resources, essentially slowing down your software. doing exactly the same thing on a mac takes less time.
here's a really brief analogy:
say you're a courier. you currently drive a ford. it breaks down a lot, you always need to buy parts for it. you have to factor this in to all of your deliveries, in both cost and time. one of your friends drives a holden. he tells you that they break down less that your ford is. you buy the holden so that you get your deliveries out faster, and you are also able to deliver more, because your van is in the shop for less time.
i'm not saying anything about fords or holdens here; i drive a toyota, and know nothing about holdens or fords. but i hope you see the point.
all these things mean that the time i spend on a mac is better spent. it's not that i can't do it on windows, but my clients are paying lots to get the job done, and i save them money, and get the job done faster (and lose less sleep, and get more work done, and win more jobs, and get more clients) on a mac.
there's a number of other reasons why i like the mac better, but these are all the reasons why any work (but graphics work in particular) is better on a mac.