I have owned both (and in fact own one of each right now by chance, though I pretty much only use the mac), and never really had problems with any version of either. Windows can be a little clunkier and buggier than OSX, but the stuff you'll be doing in med school is of such limited scope that I doubt you'll notice. They both 'copy and paste' the same, IME, and I would actually recommend mac over pc to experienced programmers rather than the other way around.
For medical school, I would recommend a ≤$500 pc laptop, for several reasons:
A) You'll be carrying it all over the place and/or living in a crappy apt and/or dorm - you may drop it, break it, or have it stolen. While it's true that mac laptops are better built, at $500 a pop, the PC laptop is easily replaceable (obviously you also should buy an external or online drive and back up all the time).
B) The programs you'll be using are only really going to be Firefox or Opera, VPN, Microsoft or Open Office, maybe a flashcard program, iTunes, and a game or two. You wouldn't be taxing any OS doing these things, nor would either OSX or windows be particularly better for such a limited range of use.
C) As weird as it sounds, you'll probably use your computer a lot less than you did in undergrad (other posters, is this true at your medical school as well?). A better machine lends no real advantage. The stuff you'll want to spend money on are a nice laser printer, an external drive, one or two high-capacity USB thumb drives, good internet access, and a couple useful online applications for file storage etc.
So, while Apple laptops are better, faster, and prettier, you'd be better off getting a $500 pc laptop every two years and spending your money on good peripheral equipment. Yes, having windows crash once every two months is annoying - but not as annoying as losing files or having your printer not work!
