A nomination is a necessity for any appointment, but it doesn't mean you are any more likely to receive an appointment. That will be approximately 6,000 nominations given out for the air force; same with the navy and army. 535 representatives and senators X 10 nominations each; plus another 500+ presidentials and other military related nominations. Of those 6,000+ nominations, only approximately 1200 will receive appointments. So while a nomination is great, because you MUST have one to get an appointment, it doesn't mean more..... Other than compared to the individual who didn't receive a nomination and therefor CAN'T receive an appointment.
Having said that; having an LOA IS A BIG DEAL. That means, if you get ANY of those 6,000+ nominations, and assuming you are medically, academically, and physically qualified; you are GUARANTEED an APPOINTMENT. That's why the academy gave you the LOA. Letter of ASSURANCE. They are ASSURING YOU, that if you are qualified and receive a nomination, then they are guaranteeing you an appointment.
You're not is a RISK situation. No yet. Just because you have a nomination to air force, doesn't mean you'll know about an appointment until probably April or May. At the earliest, maybe the end of January. On the other hand, having an LOA from Navy, you should know soon enough if you receive a nomination to Navy from your representative or senator. If you DON'T receive a nomination from them, or from a presidential or military related if you're eligible, then the LOA becomes useless and moot. Then you're back to waiting on air force to decide if you'll get an appointment or not.
The only way you'll be at risk, is if in the next couple of weeks, you find out that you DID RECEIVE a nomination to Navy. if you do; then basically you DO HAVE an APPOINTMENT to navy. Then, you have to decide whether to turn it down and WAIT and HOPE that Air Force gives you an appointment. That's a BIG RISK. If you are sure you wouldn't mind EITHER Air Force or Navy, I WOULDN'T Risk it. I'd ACCEPT the appointment to Navy. Because the air force nomination only means, you have about a 20% chance of an appointment. Get a Navy Nomination, and you're guaranteed a 100% chance of a navy appointment. (Assuming you're medically, physically, and academically qualified).
So, until you find out if you can get a Navy nomination, there's NOTHING to think about or decide. You just wait. But if you do get the navy nomination, and are offered an appointment because of the LOA, you will have a period of time to accept or decline it. You COULD contact air force once you receive that navy appointment offer, and let air force know about it. Let them know you have your air force nomination, but you also have a navy appointment offer in hand. Ask them if there's any way they can make a decision about an air force appointment for you NOW, because you prefer air force; BUT you aren't going to let a GUARANTEED Navy appointment slip buy if you have to wait until april or may.
Get the navy nomination FIRST, get the navy APPOINTMENT OFFER, THEN and ONLY THEN, can you think about options.