A favorite topic among herp forums is how people who don't know what they are talking about give advice. Some actually believe what they are saying, but that does not make it correct. Repeating "what you have heard" does not make something factual.
VoodooDragon said:
"1- Mice are higher in fat than rats.
2- When a BP is young, it needs more protein. A growing rat needs (and eats) more protein. An adult mouse doesn't have as much protein as a similarly sized rat, see? "
Both the first point and the last part of the second are completely false. VoodooDragon and many other people would know this if they did a minimal amount of research insted of making unscientific assumptions or repeating what they hear and labeling it as the truth.
The following information is based on data extracted from a paper published a little over a year ago by the USDA entitled "Nutrient Composition of Whole Vertebrate Prey Fed in Zoos".
Based on a 30 gram (average adult) domestic mouse and a 30 gram domestic rat (large pup)the following values exist:
Mouse contains 5.46 grams of protein, while the rat contains only 5, so the mouse has nearly a half gram more protein.
Mouse contains 2.33 grams of fat, while rat contains 2.475 grams, so the rat is actually MORE fatty than the mouse.
So, based on scientific data, the mouse is a better food source (they also have better values in most micronutrients like calcium due to higher bone development) than a similiarly sized rat.
However, when the rat becomes mature, after 50 grams, the values change. If you wanted to compare feeding two 30 gram mice vs one 60 gram rat:
The rat contains 12.57 grams of protein vs. the combined protein content of the mice at 11 grams.
The rat is still fattier, though, containing 9.89 grams of fat vs. the combined fat content of the mice at 4.65 grams.
Rats are more convienent to feed, easier to breed, easier to live with (all my opinions). Either one is perfectly suitable as a food source for your ball python. It's your choice.