i just got done writing a 12 page paper on a ketogenic diet for my obesity and weight management class. the general idea is that once you've used up your glycogen storages your body starts looking for other resources to fuel the body. once carbs are gone, the next thing it will look for is fat. what makes ketogenic so good is once your body starts looking for that next source of energy it will feed off the energy storage (fat) in your body. this will result in an increase in fat metabolism. you dont want to stay in this phase too long b/c eventaully your body will start using protein and lean muscle for energy. so thats why there is a once a week carb reload. this helps prevent catabolism and spare the muscles form breaking down.
having a low kcal diet, your limited to the amount of energy your body recieves from food. once this is used up it will start feeding off itself (like the ketogenic) but since there is no "reload phase" your body will search everywhere to fuel itself, and will eventually break down muscle tissue. your basically starving yourself. with the ketogenic your amount of kcal remains the same, its just coming from different macronutrients.
The argument of which diet is better continues. there have been many studies but no concrete evidence one type of diet is better than another. you kinda have to see what your body responds to the best.