For what it's worth, most of the computer nerds I know (as in, people who work in computer science) prefer Macs to PCs these days, now that Macs run off of a UNIX based platform. (Pre-OS X was a whole other story granted.) The system is simply more elegant and better designed than Windows. Whether that's worth the extra money for someone is a whole other issue, or whether they have software needs the platform doesn't meet, but pretty much no one I know who knows much about computers actually thinks much of Windows itself. They almost all would say LINUX beats either actually, but that's not particularly practical for most people.
I have a Mac laptop (a Macbook), and it's been incredibly reliable, and it was fine for the work I did my old clinical psych lab and at my current job. If your lab uses SPSS, you can get it for Mac, and files transfer across platforms fine. I've also used Parallels (the whole Windows within a Mac thing), and it works fine too, although it's a little slower, so if you are planning on primarily using Windows, I would probably just get a PC, but if you just want it for the occasional thing, then it's great.
Another good thing about Macs is that the customer service is great. I've heard horror stories about Dell and some of the other PC brands in comparison, although it's possible that I just don't hear the good stories as much.
I would talk to your program and lab and see if they have any strong preferences, and if not, then get whatever system you like the best. Being comfortable with your computer and able to work with it well is the most important thing.
