Mice have more calcium, less fat, and slightly less protein.
http://www.nal.usda.gov/awic/zoo/Who...nal02May29.pdf
For either mice or rats, anything unweaned is much higher in fat and lower in calcium and protein. So, given a mouse and a rat of the same size, it's almost certain the mouse is more nutritious. Therefore, if the pet you are feeding will never require anything more than a single mouse per feeding, it's probably a good idea to keep it on mice.
If the snake will need more than a single mouse though, it's better to go with rats due to multiple prey items being more difficult to digest. This is because multiple prey items tend to line up one after the other and thus take up much more length in the digestive tract than a single larger item. You are then only making use of the length capacity and not the girth capacity of the snake and thus will never be able to feed the ideal prey item size. Think of it as feeding a super long, skinny rat instead of a normal rat to a snake.
So, for larger snakes, the question isn't so much if to use rats instead of mice but for those started on mice, when to switch to rats. Some people feel that's it easier to make the switch when the snake is younger and thus make the switch as soon as possible.
Personally, I think the age of the snake is only an issue once it's slowed down its feeding (i.e. as it approaches sub-adult or adulthood), so I would switch from mice to rats simply when the mice are no longer big enough. I don't have a lot of experience or any literature to back this up though; it's just my personal conjecture.