Hey Sofia, to understand which one is better amon IMAP and POP3, you first need to know what are the differences between IMAP and POP3. Checkout this link: https://www.edureka.co/community/40790/whats-the-difference-between-imap-and-pop3 to understand better about the differences between IMAP and POP3.
Now about your question, IMAP is an advanced and better protocol than POP3.
IMAP is much more advanced and allows you to see all your folders. You can quickly view subjects and message bodies of emails. It can delay downloading larger emails, such as those with attachments, until you want to view them in their entirety. IMAP also allows you to synchronise mail folders between your home machine and on the web, so that you see the same folders and messages wherever and however you access your email. While POP only allows downloading messages from your Inbox to your local computer. Generally, once transferred, the email is on your local computer and removed from FastMail.