An unscientific one-month test but still worthy of reading. http://www.dailymail.co.uk/health/article-2546975/One-twin-gave-sugar-gave-fat-Their-experiment-change-YOUR-life.html A common response of bloggers to this story is that each diet would have worked better if it had been adhered to longer than a month (low-carb prononents saying that the low-carb would have worked; low-fat proponents saying that low-fat would have worked)
Low-carb diets have a much better track record for weight loss than low-fat diets. At the same time, I realize some people can do just fine on a low fat diet. People who suffer from metabolic derangement and insulin resistance would likely see better results from a low-carb diet.