I interviewed at USUHS 3.5 years ago (I'm a 4th year med student now... Actually the branch of service that you pick DOES make a difference as far as admissions goes.
From the people that I spoke with on my interview day, the air force slots fill up the fastest. This si due to the fact/perception that the air force has the cushiest bases or something like that. Navy fills second since it is not as desirable as air force. In the nay, I believe that they still highly encourage/require a GMO (general medical officer) tour after graduation and before residency that ususally is for 1 year. Army fills last due to the fact thta the army is the biggest branch and therefore has the most MD slots open.
How does all this matter? Well, if you put navy down as your preference, and all the navy slots are filled, you will only be considered for an army slot AFTER all of the army applicants have been exhausted. Therefore, there could be army slots open that you will not get considered for if you put navy as your preference, and there are still people on the army list waiting for an army slot. However, it's pretty early in the admission cycle so you will probably get offerred a navy slot.
I was actually on the waitlist, and I had chosen navy as well. I got a call from the adcom in May or June saying that they were getting to the end of the army waitlist and would I be interested in an army slot if they ran out of army applicants. Apparently the navy slots had already been filled for some time.
SO, if you want to maximize your chance of admission to USUHS, and you don't really care what branch you are in, put army down. Personally, I would rather be in the navy since the uniforms are way cooler! In any event, I did not get offerred admission and I wound up at another med school. But I loved USUHS and I would have seriously considered going there if I had been offerred a spot.
Good luck!