The weight you get from 3-4 mice and 2 rats is going to be different because there is more 'stuff' in rats. With 3-4 mice, you have 3-4 sets of skeletons to digest, and more fur. With 2 rats, you get a higher density of food rather than more bones and fur.

I don't think it would be enough to really cause problems, but if you're asking the difference, there you go.