With a small mouth-and-nose (ori-nasal) breathing mask inside the full-face mask:

Here, the dead space is less, and there is less space to needing to be blown empty if the mask floods. But the smaller space around the mouth increases the "gasmaskyness" distortion of speech.

With a mouthpiece inside the full-face mask:

Here, if the mask floods, the diver can breathe again before clearing the mask, and many British naval and frogman's rebreathers have this mask arrangement; but the full-face mask plus mouthpiece combination acts like a strapped-in gag making clear talking impossible.

Other features:

The inside of the mask window may get misted/fogged, and need to be treated with a demister. Some British Army and Russian gas masks are designed so that when the wearer breathes in, the inflow of air blows over the inside of the mask window and tends to evaporate any mist deposit on it.

The mask window should be a modern strong polymer and not glass.