Baby rats can be higher in fat than mice of a similar size. My personal preference is to start my baby balls on mice, and raise them up on them until they can feed on wean rats.
Each breeder has their own preference as to what works best. Me, I use mice. They're easier for me to get at the size I want, and less expensive. I also know plenty of people who start with rat pups. Comparing babies side by side, it is hard to say one is any better than the other, at least in my experience.
-Jen