Both formats store data, but they suit different jobs. Here is a quick way to choose.
CSV is a plain table of rows and columns. It opens in any spreadsheet and is ideal for lists, exports and simple reports.
JSON keeps nested data and types. Apps, websites and APIs use it, so it is the usual choice when software reads the data.
Make sure the first CSV row holds column names, and use UTF-8 so accents survive. Then convert and check the first few records.