Classic Email Editor vs. New Email Template Builder: Key considerations?

We're helping a client decide which email builder will be the best fit for them. Can anybody recommend a good resource that defines the key details to consider when making that decision?

What possible deal-breaking functionality is available with the classic tool but not yet with the new tool?

I'm interested in official documentation as well as 3rd party voices. Here's what I've found within the official docs:

• Considerations for Using Email Template Builder
• Guidelines for Using Email Template Builder

Thanks very much.