Headlights are clear plastics.
PlastX says "Clear plastic cleaner & polish" on the label.
The Mothers Plastic Polish label doesn't mention anything about clear plastics vs. colored plastics.
PlastX says "removes fine scratches", "Removes oxidation and grime", and "removes haze and cloudiness". It does all of the above from my experience with it.
The Mothers bottle says "Removes scratches", "Cleans, shines, and protects", and "great for headlight covers, plastic lenses and bug shields".
Another thing I take into consideration is the possible hazard contained within the product. PlastX identifies itself as a "possible eye and skin irritant", "Non-Toxic", while the Mothers label says "harmful or fatal if swallowed. See 'DANGER' on back label".
Maybe I'm one of a select few that looks at the health hazards of products to aid in my search for the "best" product for my use. I'm the same way with things like APCs and polishes. If a polish produces carcinogenic dust, or an APC warns against toxic fumes, I don't have a need for it. Although I did recently see that my Meg's APC warns against breathing its fumes, so I probably won't use that again inside, unless the windows are wide open.