Isharah yields an incidental yet text-grounded implication, such as paternal attribution implied by “al-mawlud lahu.”
Dalalah carries the legal ruling to a case indicated by the wording’s linguistically understood rationale, such as forbidding severe mistreatment of parents from the ban on “uff.”
Thus the former concerns a non-purposive textual implication; the latter, an extension through a rationale immediately evident in language.