every class is different, but my experience has been that Fitness Boxing's focus is fitness and Boxing's focus is, well, the sport of boxing. in Boxing class, you'll have a lot more focus on form. Fitness Boxing is (usually) concerned with keeping your muscles moving and keeping your heart-rate up. you don't need to throw a perfect jab, cross or hook to sweat.
for cardio-vascular training, Fitness Boxing is a very respectable routine, but if you want to learn Boxing, whether you intend to compete or not, that's what you should study. either will kick your butt, and i'm guessing that's what you're looking for. good luck, and CHEERS! on your choice.