IMO rats are more nutritious since they have more bulk/fat and fat is an energy source. The reason why most feed mice when the snake is young is because adult mice have fully developed bones and can offer more calcium, not necessarily more bulk. The bump up to rats when they get a little older is beneficial because of the added bulk/muscle and fat in the rat, meaning they can go for a longer time on one meal. And since the bump up to rats is done when the snake is bigger, the rats will be more fully developed then too, meaning more calcium for the snake.

Also, I would say going to rats would be healthier since there is alot more for the snake to digest, and will give the snake more energy. Sorry, I've had the whole Animal Nutrition class. Baby snakes are just like puppies. They need to eat more often to keep their glucose levels up and to have enough energy. If they don't get enough out of that one meal, they don't have enough glucose, and then they start getting lethargic and not responding to stimuli. Maybe UberAlice can explain it a little better since she's in actual vet school