I've heard the nutritional value of a rat is better, but Im not sure if it's true. Ball Pythons have this thing they do where they imprint on a certain food. If your snake decides it loves mice, it may refuse every rat even if you try all the tricks to make that rat look tasty. So, in
the future you will end up buying tons of mice to feed just one snake. Yours are babies now, one or two mice are affordable for the moment. Imagine how many an adult would need! My Carpet Python was eating 5 mice at a time weekly with her previous owners. It took me months to get her on rats. Since then she has filled out and glows!