As you may know, both words are adjectives, have the same defintions, and are synonyms of each other. So, in most cases they can both be used, for example: I hope you are not a negligent person. I hope you are not a neglectful person. If there is any un-written usage rule, it might be that we would use "neglectful" when it is followed by a noun, e.g. He is a neglectful person, and we would usually write/say, He is negligent (without a noun following the adjective).