Hi! High school English teacher here! They are synonymous terms - I would even argue one is not stronger than the other (for example, ugly vs. hideous; or chubby vs. obese). Try it in different sentence scenarios:
1. I am afraid (versus) I am scared
2. Don’t be afraid of me (versus) Don’t be scared of me
When trying to use this idea for an adjective, “fearful” is better (again, my opinion).
Example:
- she is a fearful person.
Note: scared (base: to scare) can be turned into an adjective, too - scary. But afraid would assume the term fearful for an adjective.
Lastly, only fearful can be changed to an adverb (description of a verb): fearfully. The others don’t have an adverb form.
Hope this helps!