If you go to the red tail boa forums, they will tell you and Im pretty sure I seen it somewhere online that rats are higher in certain areas of nutrition. I dont know if its fact.

I feed rats to my red tail simply because of the size of the snake. I switched my BP over only because 3 mice are more expensive than 1 rat.

The petstore I got my boa from had him eating mice when I purchased him. I noticed with both the Boa and the BP that the growth rate went through the roof when they were switched over..... I dunno what that has to do with though, age of the snake or nutrition. I feed rats because it costs me less and I dont have to deal with multiple offerings every meal.