in most situations it is sinonyms. if it is just "i am worried", doesnt matter what you say. but in some situations you cant say different. like "restless" it is беспокойный, if you say волнительный it means exhilarating. Беспокоиться more means that you worry about it. You can "волноваться" thinking about beautiful moments in your life. its a broader concept. hope im rights))