The only real benefit to feeding rats over mice is if your snake turns out to be a picky eater. Swapping from mice to rats later on could be difficult, and feeding 3 or 4 mice at a time would suck. Nutritionally they are essentially the same. I should add that for a boa I doubt it would matter what you give her, mine switches between live, ft, mice, rats, hamster....she pretty much eats anything. Just my BPs that i dont stray away from rats.