Why use MAC
By enlarge Mac's are able to work with far less I.T. support, once they are set up properly and barring hardware failure they just go, Unix doesn't seem to attract any script kiddies, at this point in time there are no Mac Viruses.
most agency's and customers invariable work on a Mac platform as this is still the industry standard, as you are probably aware this means that you will not have any font compatibility issues that you will have if the client uses PS or composite fonts where the PC will only see the resource file and therefore on the Mac platform you will be able to work from packaged documents
The Pc, using XP/vista/7, is limited to use only 4 gig Ram for any application, the Mac uses whatever Ram you assign to a software package, the theoretical limit for the PC is 128 gig of Ram but this would require both the 64-bit version of Windows as well as a 1KW power supply with extra memory risers
Mac has on board Fiery port for target boot, this allows the user to retrieve data when the OS fails to start.
Mac everything is true 64 bit (10.5 up) Windows will run 64 bit but all the software and OS needs to 64 bit specific, the Mac can run 64 bit on a 32 bit system, if you have 32 bit software you are able to run on a 64 bit Mac, by default 10.6.4 will boot in 32 bit
All drivers are included in the OS, no need to hunt for printer or port drivers
run windows programs through vm ware.
Open GL acceleration for photoshop example when the video card crunches the data to do a screen redraw
Why use PC
Far less expensive, because in the PC market there are far less expensive options available
Much easier to replace components
Far easier to automate workflow e.g. robocopy etc
Assign multiple domains and nested domains, Mac server runs only 1 domain
Its all about using the right tool for the job, any scripting etc i use a PC, any graphics i use my Mac
however i would not upgrade from XP to 7, i would probably use Suise or another distribution of Linux