Should I use the 'New Entry' or the 'Contacts' block in my Flow?

Now, that's a good question! These two blocks can result in a quite different scenario, depending on the purpose of your Flow. Let's take a look at some scenarios:
(assuming you're already familiar with Flow, the New entry block and the Contacts block).

When to use the Contacts block

• if you have a fixed or a final list of contacts in a segment. For example, you have the exact number of attendees and you would just want to send that specific number an email. Or, another Flow has successfully finished and you have the number of contacts coming from that Flow.
• when you know that the number of contacts in the selected segment will not be updated any time soon.
• if you are using only Contacts blocks in a Flow, consider the Flow to have "closed doors" for new contacts.

Note: if you are using a segment in the Contacts block and your Flow is live, you cannot Freeze the flow to change the segment into a different one. Adding more contacts to an already-live flow is made possible with the New Entry block.

The number of contacts in a segment is locked at the moment of Flow activation. Meaning, you have a Flow with a Contacts block that has a segment of 100 contacts. You activate the Flow, and an email will be sent to these 100 contacts. If you at a later point add 20 more contacts to this segment (after the Flow activation) - the email will not be sent to the newly-added 20, because the Flow was activated when the segment only had 100 contacts.

When to use the New Entry block

• when you're uncertain that the list of contacts is fixed and final, but would want to update it with new contacts.
• when you know that you would constantly need to add new contacts to that Flow.
• if you have a New Entry block in a Flow, you're leaving an "open door" from where new contacts can come in.

Note: Since a New Entry is literally an open door, there's a possibility where a contact could come in two or more times in the same Flow. To prevent this, connect it to a Contact match block with the setting the contact has been here before, to make sure only new contacts come in and not the same contact twice. The contact matches a segment setting is another way to make sure only the right contacts are let in.

Letting new contacts in is done through the contacts in the Contacts app.

Summary

In short, closed versus open doors. Fixed versus updated lists of contacts. Find the one you need, and continue with that beautiful Flow!