Really, rats offer more fat content and that's about it. The main benefit to rats is that you don't have to feed more than one at a time, as rats can get more than big enough for your snake. Mice only get to 20-30 grams, about the size of a rat pup, so if you have a big girl that's a determined mouser you might have to feed her 4 or 5 mice in one sitting. More expensive, and much more of a hassle.