I would probably do AP Spanish if you’re applying to more competitive schools. Colleges generally view AP Psych as a “fluff AP” (easy to do), but AP Spanish is regarded far higher. Of course, I would tell you to take AP Psych if you’re going to do psychology, as that’s related to your major and shows colleges you’ll follow through with it, and can handle the coursework for it. However, AP Spanish is incredibly useful for almost any student; learning how to speak Spanish can help you translate for a patient who doesn’t speak English, if you want to be a doctor; it’s useful in immigration law if you want to be a lawyer, and so on.
Basically, choose whichever matters more to your degree. If AP Spanish or Psych aren’t relavant to what you want to study, or you are Undecided, I would say take AP Psych, you’ll probably have some fun with your friends, and fond memories of senior year.