I write all the lecture notes on my Mac using Pages and OmniGraffle. I do my presentations on a Mac using Keynote. I work as a psychotherapist in addition - I do all my leaflets etc on the Mac using Pages, and run my practice on the Mac using iCal, TimeLog and a Swiss accounting system (Loops). I train reading CT and MRI scans using OsiriX - you will not get that on Windows at all. It does all your favorite PACS workstation does in your hospital's MRI (including 3D/4D/5D etc). You can load MRI scans on your iPod. And it is free. Check at http://www.apple.com/downloads/macosx/ . For fun I use it too with GraphicConverter for photos, iMovie and FinalCut Pro for movies, iTunes for music and training files, Amadeus II to edit sound files, …
The only times I do need Windows are the training CD's that sometimes were not made for Mac OS X. But Parallels Workstation takes care of those - or BootCamp, but usually it is much easier to have Windows running under OS X, and performance is very good (say at least 80% of native speed unless you need high performance graphics - then Boot Camp is the way to go, and your Mac Book Pro will be as fast as any of the best other Windows laptops.)
Bottom line: Get a Mac, and buy it as late as possible (cheaper). Enjoy!
HTH
Marc