There are a few different arguments I have heard. One is that it can be pretty inconvenient to feed mice to an adult bp because they quickly reach a size where one mouse will not be enough. Rats get bigger so they can be kept on rats their entire life, just keep increasing the size of rat as needed. Because bps are already known as picky eaters, it can be hard to switch them once they are used to something. Maybe if you were to feed both somewhat regularly you would minimize the transition if you ever do switch to only rats as your snake outgrows the mice. Feeding rats is just cheaper and simpler when they reach adult size

As far as nutrition goes, I have also read the argument that rats are more nutritious than mice. There are differences in content between rats and mice, but whether these are significant enough to really be an issue is still up for debate. Many snakes have lived our their entire lives on mice (usually because they refuse to eat rats) and are perfectly healthy, so the issue is really what is easier for you as the snake is going to be fine either way. Most people seem to agree though that getting them used to and keeping them on rats is the way to go.

My advice would be to find out what the snake has been eating when you get it. If it is on rats I would consider that fortunate and keep it that way. If it is on mice I would still say to try offering a rat and see if it is willing to eat it, and continue feeding rats if it is. If you do wind up with a snake that will only eat mice don't stress it, but always be thinking of care requirements down the road and try to set things up to have as few problems come up later as possible.