Flat or contoured?
How the two shapes differ, and who tends to prefer which.
The flat mask is two layers of silk with a thin layer between. It's light and easy to pack, and it sits close to your face.
The contoured mask has shaped, lightly padded cups, so it doesn't touch your eyelids. Some people find that more comfortable, especially with long lashes.
If you sleep on your side, the flat mask's lower profile is usually easier against a pillow.