I have used both in school...I have a Dell XPS m1210 and a 13" MacBook (the black one). I used my Dell all of 1L, and my Mac in class all of 2L, but I always took exams on the Dell because the tech people seemed to know how to quickly resolve any PC issues much better than they did Mac problems.
My Mac was fine my first semester of 3L, but then the first day of this semester, Microsoft Word crashed about 6 times and I lost all my notes. This then started happening to everyone with a Mac. The school says it has something to do with their network, I have no idea. So now I just use my Dell....for the bar (at least, the NY bar), you either need a PC or an Intel Mac with XP or Vista on it, so you might as well get used to the SofTest exam program beforehand.
The bottom line I have discovered is that laptops mostly suck, no matter what brand you buy...law students put a lot of wear and tear on them.