The coating INSIDE the lens is not usually desirable, however you are correct about the outside. If you get a mask that has some sort of "underwater colour enhancing" properties, DO NOT scrub the outside with anything abrasive, even toothpaste!Scuba mask design is constantly changing. Today, some manufactures are including a lens coating on the inside and outside of the lens. Some masks come with anti-fog coatings and color filter coatings. The coatings will of course wear down and eventually wear off. The life of the coatings can be prolonged by using very mild detergent, no abrasiveness, and minimized rubbing.
The primary thing that causes fogging is particulate matter (dirt, make-up, etc) on the lens. A clean lens is very unlikely to fog.
A sneaky lens contaminant is facial make-up or any material used on the facial skin. Materials on the face will transfer through the water within the mask and attach themselves to a clean lens. The lens will then begin fogging.
Personally, I like to avoid those... stick with the clear glass, then, in a pinch, it can be defogged underwater with a handful of sand.