The fitness benefits of DDR are old news, in the Wii is new so that's probably why you're hearing more about the wii.
There's some interesting articles posted over at ddrfitclub.com about using the ps2 and ddr for exercise.
Having played wii sports and ddr, I find it impossible to play a higher level of DDR without breaking a sweat, whereas I really have to try to get much of a workout from wii sports. The benefit to a wii is that as more games come out for it, you'll have different methods of exercising and interacting. Whereas there are only a handful of motion games for ps2.
I plan on picking up a Wii in a few months, hopefully by then they will have annoucned DDR for Wii.
Going to the gym gives you a more even, intense workout, but DDR is great for motivating yourself since it doesn't feel like work. Any excercise is better than nothing, so if DDR is all you feel like doing, it still beats the heck out of sitting on your butt in front of the TV. But if you're really looking for results you'll want to start running, cycling, swimming, etc.