How do you know that rats as oppose to mice offer more nutrition? There has never really been a study on the nutritional requirements of ball pythons. I don't know about being meatier but rats do have a higher fat content which would explain why bp's seem to gain weight faster on rats than mice.
The rats are better than mice therory is just opinion not really based on any fact.
I fed my bp 3 mice a week for three years and his vet, Dr. Scott Stahl, said he was one of the healthiest bp's he had ever seen.
I think you should feed what your bp will eat. The key is not so much WHAT to feed as to how much/little and how often.