Which service has the best officer (RN) promotion rates? Or where could I find info on this?

I'm an AD nursing student (NECP), and one of my classmates (civilian) intends to do at least a 4-year stint in the service to continue schooling. He's thinking Navy, and I'm thinking about his promotion potential, in case he decides to do 20…

Where might he be most likely to pin on an O-6 or above?