Its quite a suprise to here that your a graphic designer and have not used a Mac.
Over here in the States Mac has been the computer of choise for most of the graphics, printing and photographer world. Its about the only real niche the Mac has.
I have been using a Mac my whole profesional career. I find it much more intuitive that PC's
I'm a creative person I don't want to spend time figuring out computer tech problems. Mac makes my job easy and keeps me focused without having to deal with computer problems.
One of the best pages I have seen that talks about the Mac advantage is right on the Apple web site.
http://www.apple.com/switch/
Read it! Every point is true and correct
Then if your still not sure go and try out a Mac, you will love that new 17 inch powerbook.