Check out this study and decide for yourself.

http://www.nal.usda.gov/awic/zoo/Who...nal02May29.pdf

This study offers some interesting statistics about the notion that rates are better than mice as a food source. I've read many posts saying rats are more "nutritious or healthier" than mice. From what I've read in this study, and speaking very generally, I've gathered several contributing factors to whether rats are really better than mice as a food source.

1) The food that the rat or mouse was raised on will contribute to its overall nourishment for the snake.
2) The age of the rat or mouse at the time of consumption. As the rat or mouse ages from one stage to another there are greater or lesser degrees of certain nutrients.
3) Rats and mice metabolize some nutrients differently-Regarding vitamin E, "Clear species differences in vitamin E metabolism/utilization have been shown between laboratory mice and rats raised on the same diets,7,15 suggesting that species that have not been studied may have different requirements."

The tables provided at the end of the study at times indicates wide variation and at other times very small differences in nutrition, again depending on the age of the rat or mouse. It would seem to me that from this limited study, it would be hard to support the argument, across the board, that rats are nutritionally better than mice. Having said that though when it comes to convenience, cost, and keeping the python feed, it only makes sense to feed larger items, i.e., rats if possible. In My Opinion, they are both probably offer equal nutritional. We as purchasers of live or frozen rats or mice usually have no idea what food they were raised on, how they were euthanized, etc., and therefore have no idea what's under that fur coat. I think you have to look at what your buying and judge as best you can on appearance and behavior of the rat or mouse. (In the case of frozen, behavior is usually consistent, LOL). Initially, I don't think it matters which you serve up. When the snake gets larger, I would go for rats for the reasons mentioned above.