If you're thinking of breeding rats or mice, I would suggest both if possible. The mice are great, because you never have to worry about them getting too big (unless you're feeding hatchlings, in which case you would use pinkies). And the rat pups you can use, if you're low on mice. Also, you can let the rats get bigger for larger ball pythons. I typically have many mice on hand, and only a handful of rats. Thier diets are slightly different though, so if you prefer more protien in your snake's diet, rats are the way to go.