Which are more fattening, foods high in fat or those high in sugar?

I understand about calories and that if you consume more than your body needs you gain weight. I also know that foods high in fat and carbohydrates (sugars) are energy dense and can lead to weight gain. But I was wondering which foods are more fattening those high in fat, or those high in sugar. For example a weight watchers choc brownie dessert contains 3.3g fat and 21.9 carbs in the form of sugars, whereas a pack of fries to go has 12.8g fat and 0.5g carbs in the form of sugars. Which would be worse for making you gain weight?