Buff sounds like the best idea because it's not going to be too bulky, it won't make you sweat as much and also, weird as it sounds, the silky/shiny texture of it will help the helmet slip on to your head easier stopping your hearing aids from snagging.
I'm soooo far out of touch with full face helmets that I can't even remember the name of the one I used to wear!! But what I do remember is that you really want it to be a snug fit, there's nothing worse then a full face that's slopping around on your head. It's pretty dangerous especially if it flicks forward of a drop and covers your eyes!