[kahljorn]

tae kwon do would be better for exercise probably like mock was saying(i was really flexible when i took it too it was great to be able to kick people in the face). You do alot of kicks and punches and sit around doing it all day usually, lots of cardio.
Judo is grappling, it has a lot of throws and takedowns etc. if i remember right it's also somewhat difficult because it's supposed to be flowing/fluid. You probably wouldn't get much (or as much) exercise doing that. You'd probably learn how to beat someones ass best in judo though.
Although learning the sensitive parts of the human body in taekwondo is cool. I remember we had this dummy that had the points on the body that will cause the most damage if you hit them marked. That was kind of fun.
However, most martial arts instructors will make you do some exercises though, regardless of whatever the techniques are.

if you want good exercise I'd also reccomend you take samurai sword lessons if you can find them. That shit is sohard. There's moves where you're sitting on your knees (legs are tucked under you) and you have to jump up and attack from that position. It's really hard, just try to do that. Other than that using a sword is pretty damned easy once you understand it's fluidity and how it cuts/works.