1. "Intérêt": This is the most common and general spelling, used for the concept of interest in general, like "interest in a topic" or "interest on a loan".
2. "Intérêts": This is the plural form of "intérêt" and refers to "interests" in the sense of individual preferences or advantages. For example, "conflit d'intérêts" (conflict of interests) or "les intérêts nationaux" (national interests).
So, the spelling depends on whether you're referring to "interest" as a singular concept or "interests" as a plural, representing individual preferences or advantages.