Coverage

The main areas of coverage on the headgear are the forehead, cheek, sides, chin, and the back of the head. Different headgear will vary in their coverage.

The cheek and forehead are the most important areas because that's where you'll be hit the most, AND they affect your vision. The sides of the headgear are also crucial because you'll be hit there often. The side doesn't affect your vision but you don't want it too thick because you'll feel like your head is a giant target.

The back of the head will vary in amount of padding for different headgear. Wide punches often land there so be aware of that if you have a boxing style that takes many punches to the back of the head. (ie: lots of rolling).

I've found headgear with chin padding to be incredibly annoying. The chin padding makes it difficult for me to lower my chin and doesn't seem to cushion my chin at all. Some headgear also have a facebar which is a bar going across the nose). I've seen many pros sparring with this headgear and enjoy its level of protection. I think it's great if you have a nose that could break but otherwise the facebar makes it difficult for me to see punches during sparring.

These facebar (aka "facesaver") models are commonly used by competitive fighters to prevent breaking their noses or getting a bruise/cut before a fight. Keep in mind that some of them protect the nose more whereas others may have the bar lower to try and protect the chin (as well as increasing vision).