While I am not 100 % certain, MO's, Nurses and other professionals get to choose the uniform they wish to wear. From my experience its like a 45/45/10 % split for MOs. The ones who choose a navy uniform do so because it is the sharpest looking of the three. Air Force has the smallest percentage because there is only one or two Doctors on their bases.
For nurses it is pretty much equal, but the do get asked their preference.
For the trades, you may be given a choice, but more likely, you'll just get told what uniform you are wearing, based upon a percentage of intake.