I'm an American who plans on attending Windsor in the Fall. That said, I find it really wierd that a "part-time" program at an American law school is 4 years and that the extended option at Canadian law schools is up to 6 years. Both fulltime are still 3 though. I dunno, that doesn't answer your question but it'd be nice if someone could explain on top of that.
However IMO, you should do the fulltime option if you're seriously considering a whole new career change into law. If you were just doing it for the extra degree, to work in a different dept at your company or for promotion reasons, the part time would be for you.