I have a little tweak to your test that you could try... Since we tend to think that rats are "more nutritional" for snakes than mice you should be able to feed a smaller rat than mouse and still get the same results... Like adam said youd need a scale to weigh out the rats but feed more mice and less rat and see what happens... If rats are more nutritional you should be able to feed less and get more... I dont think it makes a difference, mice or rats as long as your snakes eating something.