Long, slow cardio burns more calories during training, but interval training burns more calories overall (during and after training), because it makes you burn calories between training sessions since your body must recover from the intense bout. It's "hard" on your body. Winner: interval training.
Long, slow cardio won't help you gain muscle mass. On the other hand, interval training makes you gain muscle mass, because to sprint or bike like crazy you need to push on the pedal really hard. This builds muscle (think of a sprinter). Since the more muscle you have, the higher is your resting metabolic rate, interval training makes you burn more calories all day, every day. Winner: interval training.
Another benefit of interval training is that it takes much less time (about 30 minutes per session). Winner: interval training.
On the other hand, interval training is too hard for beginners. If you're a beginner, stick to slow jogging first for two weeks, and then experiment with increasing the pace for 5 minutes and walking for the next 5 minutes. Once that becomes easy, shorten the work interval and increase its speed. Winner: long, slow cardio.
Overall, if you're fit, interval training is best. But since it's very hard on the body, no one (even elite runners) does it more than twice per week. So, to lose fat quickly, your best bet is a combination of interval training (1-2 times per week) and long, slow cardio (2-3 times per week). Plus it brings variety to your training, which is good. Winner: both.
To sum up, if you want to maximize fat loss, I suggest you do interval training alone on the days you do it, and that you do some weight lifting and then long, slow cardio on the other days you train.
This being said, the specifics of interval training can get tricky. Hopefully I can share with you the method I prefer later. Athletes and smart trainers use interval training: you also should.
