Inspite of the similarity in names, boxing and kickboxing are wildly different animals. You may need to ask more questions or actually go there to see which one they actually do at each of these clubs, though I wouldn't be suprised if a couple of them offered courses in both.
Since you're looking at martial arts primarily for fitness, I'd suggest going with a kickboxing course. Many of the long-time boxers I've seen have massive arms with tiny little stick-legs. It's not so much that their legs aren't in good shape, as it is their arms are in way better shape, and it creates a stark contrast that I, personally, find a little comical.