Which one is better POP3 or IMAP when using with Outlook?

Answer 1

Use IMAP when possible. An IMAP connection keeps the mail on the email server, making it accessible from multiple devices and web-based clients.

POP3 downloads the messages to your local computer and removes them from the server. Your messages would not be available on multiple devices when using POP3.

This is true with whatever email client you are using and is not limited to Outlook.