http://www.rodentpro.com/qpage_articles_03.asp
So you can know how much crude protein and magnesium you are eating in your next meal of tadpoles. Oh and it shows rats and mice too. Personally I love a good stew of gecko, prairie dog, and shrew but we can't always eat so well.
In all seriousness, there is absolutely zero nutritional difference between rats and mice except that rats can get bigger than mice. That's all.