To just state the conclusion: if you’re not sure, use module.exports.

The Node.js official documentation says the same thing.

As a guideline, if the relationship between exports and module.exports seems like magic to you, ignore exports and only use module.exports.

• Source: exports alias - Modules Node.js Manual & Documentation

If you want to know more about the difference between module.exports and exports, you might want to read the following articles:

• Node.js : exports と module.exports の違い（解説編） - ぼちぼち日記
• [NodeJS] モジュール定義について学ぶ - YoheiM .NET
• Node.js - exports と module.exports の違い - Qiita

That’s all from the Gemba.