It depends on your DNS and server configuration:

• If one redirects to the other (e.g., http://example.com redirects to https://www.example.com): You technically only need to monitor the final destination (https://www.example.com) to know if your site is online. However, adding both is recommended to ensure your redirect rules are working correctly.
• If they serve different content or do not redirect: You should monitor both to ensure both versions are accessible.