Jossh,
Vey little difference (you can search at RodentPro.com) in nutrition, but there is some.
Most (most) keepers stick with rats since it's easier to feed on small rats than 2 adult mice. (Time, cost, etc).
As previously mentioned it becomes harder to swap out mice, for rats, with age of your snake so if you think you want him on rats down the road switch to rats now.
This morning I watched a youtube vid of a wild adult king eating a Rattkesnake. (Slo-mo was amazing.) So if you are happy with mice, stay happy. If you find challenges switching to rats, from mice, let us know. (There are tricks).
Heath' & Chris