Jase, mice and rats are nutritionally equal, except rats are a tad more fatty. Adult mice and adult rats have the same calcium levels. Just like a weaned rat and a weaned mouse have the same nutritional qualities, just one is bigger than the other. And yes, as said above, it is recommended to just feed one prey item that is 1-1.5x the size of the widest part of your snake. In multiple prey items there is just alot more surface area than a single prey item. More surface area= more undigested hair, and other material that the snake just passes anyway. So why not just feed one item so there is more digestability.

Just my $0.02