Having been a makeup artist for 14+ yrs, I agree with a lot of the posts, siding on the side of makeup. There is a place for masks, but I have nothing new beyond the scenarios already mentioned.
I do have to say, that even if you're scaring in lowlight, you can do makeup that would enhance the experience and it make it scarier. You would just make the contrast of highlights and shadows more defined, which would distort the human features even further. However, if your makeup dept isn't skilled enough to do it right, it could blow up in your face.
It may be to your overall benefit to get your artists some training so they can be quick and good.
As for masks, the really expensive ones can be pretty sweet, but sweaty!
In my experience, the majority of houses that use masks, seem cheesy and the customers don't "get" it. There also tends to be a lax in attention to detail, i.e. exposed skin at the neckline, no black around the eyes, pink lips and mouth...
Just my opinion!
Do Vegan Zombies eat heads of lettuce?