When I tried a FF mask, as my mouth opened, the air seemed to rush into my mouth rather than rushing out. Since the pressure is the same against the nasal openings as against the mouth, this makes some sense. This usually woke me up and caused extreme dry mouth.
With my nasal mask, the air always rushes out when my mouth is open but usually doesn't wake me. Whenever I have a seriously dry mouth in the morning, I know my mouth was open at some time during the night.
Before I was successful in sealing my mouth with my tongue, I tried taping my mouth closed a couple of times with my nasal mask, so that all observed leaks were mask leaks. They were much lower than mouth leaks and and were intermittent. My mouth leaks are always sudden and much higher (35+). When my mouth closes, the leak disappears just as suddenly as it appeared.