Char, I think you will just have to try it to find out. I don't have a particular problem with dry mouth, but the tendency is greater, for me, with a nasal mask. It depends on the cause. Any mouth breathing, in or out, will tend to be drying. With a Nasal Mask, you could be experiencing mouth leaks, which we've discussed before, where air is being rather forced out of your mouth by the CPAP pressure. A full face mask will tend to prevent that because it will equalize the pressure at the mouth and nose. However, even with a full face mask and CPAP, an open mouth could be drying. And breathing (inhaling and/or exhaling through the mouth) definitely would. To keep the mouth closed with a FFM, some still resort to a chin strap or tape. However, If you do switch to a FFM and experience drying, I suggest trying a modified chin strap which will serve as more of a reminder than a preventer. This type of chin strap you would make yourself. It would simply be a small strap that runs from one lower strap of the mask, under the chin, to the lower strap on the other side. It could be made of stick to itself Velcro. This strap doesn't have to be tight, just snug as your mouth is in a naturally closed position, which would become tighter if your jaw drops. Along those same lines, some have used tape in place of the strap. One piece on each side from the mask to just under the jaw line. I don't really recommend a full blown chin strap or mouth tape with a FFM.
So unfortunately, regardless of the experience of others, the only way to know for sure is to try it yourself. Hopefully, Pugsy will have one to trial.