Sprints or interval training has a much more anabolic hormonal effect. Anabolic meaning building, growing, and synthesis of cells. This is a much better environment for your body post workout than being in a catabolic state. Catabolic meaning the breakdown of tissues, cells, and use of energy. With intervals there is a bigger increase in growth hormone (GH), insulin-like growth factor 1 (IGF-1), and testosterone (T). These anabolic hormones will promote muscle gain as well as fat loss resulting in a leaner versus skinnier look.
With sustained aerobic activity there is little increase in anabolic hormones. In addition to this, there is a much bigger increase in Cortisol, commonly referred to as the stress hormone. Although very much needed, Cortisol can wreak havoc on the body if it stays elevated in excess magnitude or duration. This makes sustained aerobic activities like distance running very catabolic and stressful on the body. The result is a decrease in both muscle and fat if weight loss occurs.
So if you want a better shot at decreasing your body fat percentage perform intense intervals for cardio. If you want to be smaller and flabbier then keep doing the same boring 30-45 minutes of steady-state cardio.
1) Marcus C.C.W. Elliott, Phillip P. Wagner, and Loren Chiu, Power Athletes and Distance Training, Physiological and Biomechanical Rationale for Change. USC Sports Med 2007, vol 37, issue 1. Retrieved from: http://www.spartascience.com/publishedresearch_assets/Power%20athletes%20and%20distance%20training%20physiological%20and%20biomechanical%20rationale%20for%20change.pdf
2) Tremblay A, Simoneau JA, Bouchard C. Impact of exercise intensity on body fatness and skeletal muscle metabolism. Metabolism 1994; 43 (7): 814-8 Retrieved from: http://www.sciencedirect.com/science/article/pii/0026049594902593
3)Shawn Talbot, PH.D., FACSM (2007) The Cortisol Connection - Why Stress Makes You Fat and Ruins Your Health - And What You Can Do About It.