Rats are larger so you can't start a hatchling corn on them. And if the snake is feeding well on pinkie mice then there isn't really a reason to switch them to rats. I have also heard that rats have a higher fat content. So, the snake will grow faster but it won't be muscle...its just stored fat.
I'm not sure I agree with the whole fat thing.
My snakes have great tone, and they grow a LOT faster with rats as compared to mice.
As far as fat content, it totally depends on the rats and the mice.
If you raise skinny mice and fat rats, then you'd be right.
But if you feed skinny mice to your snakes, your snakes will be anemic, and always hungry, which isn't good either.
Moderation in all things.
If you feed them too much fat, you'll smell it in their waste, it will be foul smelling and too runny. My snakes get a mix of rats and mice, and their waste is firm enough and smells like it should.
I prefer rats because I raise my own, and it's much easier, less smelly, and you get much faster growth rates in the rats (From day old to feeding size in 2-3 weeks.) and snakes grow much faster when fed a diet of rats. (There's more meat on a rat than a mouse.) Mice are all bones, hair, and water.