PartialViewResult and JSONReuslt inherit from the base class ActionResult. so if return type is decided dynamically declare method output as ActionResult.
public ActionResult DynamicReturnType(string parameter)
{
if (parameter == "JSON")
return Json("<JSON>", JsonRequestBehavior.AllowGet);
else if (parameter == "PartialView")
return PartialView("<ViewName>");
else
return null;
}