Is there a rule to decide which is better: relevant to or relevant for? One is accusative and one dative but that doesn't really help me.
Where did you get the idea that this would have anything to do with the accusative and dative cases? Objects of prepositions in English are commonly said to be in the objective case, which also includes direct and indirect objects (nouns, pronouns, gerunds, and noun phrases).
That said, to is the preposition most often used with relevant, though there are some instances where for might be used:
The medical advice given in the article was relevant for many sufferers of tinnitus.
Here the object of relevant is not stated; it is merely implied, and so the preposition for now links the adjective to a group of people who would find the article relevant. Had the object of relevance been explicitly stated, the preposition used to link it would have been to.
The medical advice given in the article was relevant [to procedures that can provide relief] for many suffers of tinnitus.
The above fills in information that went unstated (by means of ellipsis) in the first sentence.