Welcome aboard Jules! Every diet usually works better the first time. One of the reasons is yo-yo dieting, where your body becomes more resistant to weight loss. In addition, lowering calories slows the metabolism. But metabolism can be rebuilt with frequent meals, and exercise.

As for which diet, you've read both books, chose the one that makes more sense to you. You can always change. But keep in mind that 3 weeks is not enough. Give it more time, and make sure you take your measurements, this will ensure that you notice body changes even when the scale is the same. This can hapen when you gain muscle and lose fat, your weight might be the same, but you are less fat.

I look forward to hearing about your progress!

Wa'il