Mice or rats, one is not better than the other. It's about volume of food. An adult doesn't need anything larger than a small/small rat once a week, which is approximately the same size as two large adult mice. I raised my current breeder females on mice, but I do have everyone on rats now, simply because I now breed rats. When they were mousers, I fed them two mice a week.

Feed what's most convenient for you - they'll grow fine on either.