Exercise Intensity vs. Duration
It seems like there are more bits of exercise advice floating around on the internet and in books than there are stars in the sky. The fact is that what works for one person may not work as well for another. Of course the bottom line is that it's better to do ANY exercise than none at all. However most people are looking for workouts that will allow them to get the most "oomph" from a set amount of time, and will allow them to make the fastest progress.
I believe that to make progress faster in regards to losing fat and increasing your metabolism, intensity trumps duration.
In other words, if you pack 30 minutes of exercise into 15 minutes, in the long run the 15 minute routine will give you faster progress than the 30 minute routine, although technically both will have burned the same amount of calories.
I'm not a physioligist, but I've read enough and also experienced personally the effects of high intensity exercise. The fact is that working at a higher intensity level ramps up your metabolism and keeps it at a higher level for a longer amount of time. In other words if you do a slow jog for two miles and it takes you 20 minutes vs. doing a series of sprints interspersed with your jogging that takes 10 minutes, you will ramp up your metabolism more with the shorter workout than with the longer version.
Technically speaking you've burned the same amount of calories, but I don't believe your body will agree. :-) It will *feel* like you did more work and your body will respond accordingly.