Regardless of if rats are more "nutritious" than mice, there have been no studies done on the nutritional requirements of Ball Pythons.
So! Whatever the snake wants to eat, mice or rats, feed that. They grow equally well on either. If my 35 didn't do well on mice, I would do something different. But, since they are growing at a very fast pace, and one female is even past her pre-breeding weight of last year(on a whole 3-4 mice a week, backed down to 2-3 mice a week, because she was getting too chunky), they will stay on mice. On the plus side, mice don't outgrow the snake, whereas the rats outgrow the snake in about a week or two.
Just feed what the snake will eat, and what is most convenient for you to get, and you'll be good to go.