When I think bondage hood, I think a tight leather (or rubber) hood laced snugly over my head. Whereas I think of a mask as more loose fitting and typically worn for its external appearence rather than for the sensation provided to me, the wearer. I'm sure some people would disagree with my definitions, so let's keep this little definition between us and just use it for the purpose of this article? Great, thanks!
Anyway, both masks and hoods provide a degree of sensory restriction and most will restrict (or at least give the wearer the feeling of restricted) breathing as well. In ponyplay, masks also (typically) serve the purpose of making the pony's head look like a horse's head. Indeed, some masks (e.g. those made by Fury Fantasy) are quite realistic, and a bio-horse bridle can be fitted on the mask for that final touch of realism.
Some bridles have attachments for "muzzles" or other partial face masks which cover the nose and mouth and provide a more equine shaped nose and face. While these do not provide the same appearence as the full face pony masks, they do provide a nice look and also add a degree of bondage and can provide the sensation of restricted breathing without really restricing breathing to a great degree. One example of this style is the JG Leathers pony bridle and face mask. I've used this bridle and face mask on quite a few occasions and can attest to the fact that while wearing the face mask, even without any exertion, the air heats up within the mask fairly rapidly and makes me feel very restricted.
The closer fitting the mask, the more the feeling of restriction. Thus for more extended ponyplay scenes, or scenes in which the pony will be exerting itself, it might be more appropriate to use a looser fitting mask. A tight fitting hood is thus likely not appropriate for anything except a photo shoot or a casual indoor scene.