Having your hips high like that makes you less stable and you don't apply as much weight. Your legs are actually supporting much of the weight. Try doing both against the mat and you will notice that its easier to hold with your hips down because most of your weight is against the mat. With your hips high in "downward dog", your legs will start getting tired because they are supporting much of the weight. Ask a training partner to do both and I bet he will feel way more weight with your hips low.
