Just do what works for you. If it is easier for you to find mice, then feed mice. The main reason that people like to feed rats is that you only have to feed them one. One thing to keep in mind is that if you continue to feed your bp mice, it may be very difficult to get him to feed on rats later if you decide to switch.
I don't go by weight. I measure the size of the prey item by the thickness of the snake.