well your doc is right, you will need a FF at some point.
But you will get the BEST therapy via nasal mask, or that is my experience with it.
With a nasal mask there is no moving parts like your mandible or lower jaw. Maintaining that seal can be a PIA.
If you can eliminate congestion and control mouth breathing you'll feel better from a nasal. It is so easy to push a FF mask out of kelter where it leaks, when it leaks your therapy goes out the window and not down your throat.
My suggestion:
1. Address your chronic nasal congestion, for this therapy to be successful, you need to be able to breathe through your nose. The nose is there for a purpose, to control the temperature and moisture of the air entering your lungs. Breath through you mouth and you by-pass that benefit.
2. Do NOT use any OTC sprays or steriods. Use only a saline rhino rinse. Go see an ENT and have your nose checked for a deviated septum, if you have one get it fixed you will never regret it. Then you will be able to breathe through your nose. Use BreatheRight strips under your mask, they really help keep you open combined with the saline rhino rinse.
3. If you try a nasal mask, get it larger, the top needs to land in alignment with the center of the pupils of the eye, then it needs to be long enough to reach your upper lip without cutting into your nares. Don't let them fit you with one too small, it can put pressure on the outside of your nose and lead to congestion.
A sleep lab is probably the worst place to fit you for a mask. They will fit you with a mask smaller as opposed to larger because the smaller one will leak less, and for their tests that is all they care about stopping leaks as it throws off their titration. But when you get home it is a different story, you want something that is half-way comfortable and doesn't leave a divot across your nose to wear around the next day.
someday science will catch up to what I'm saying...