I'm a mac user, and my exam software only works on Windows. Boot Camp lets you run either Windows or Mac OS, but it seems like people who installed windows on their computers had major issues. I have access to a spare PC, which I use when taking exams. There are rumors that next year the exam software at my school will work on Macs, which is why I haven't run out to buy a PC.
I would recommend waiting until you choose a school, and then seeing what your law school uses for exams before making a purchase. If you're looking to exchange, I guess you might have a time barrier to consider, but this is probably the safest option. I am still highly loyal to my Mac, but I know how to use windows. I do not see how my personal preference will have any bearing on my legal career. Most people in my section use Macs. I just wanted to let you know that I have encountered barriers.