In what is this module different than simplenews?

Constant contact's email marketing service provides a lot more tools and sophistication that Simplenews module does not have yet. If your needs are very simple, Simplenews will probably do. But if you need more sophistication and lot more scalability (since constant contact will be sending emails on your behalf) you may want to explore constant contact.

I agree. Simplenews is a Drupal-internal solution, while this module provides the ability to integrate with Constant Contact, a service provided externally. In general, I would recommend using simplenews, though. I just had a client that needed the integration, as they had a ton of lists that were setup with Constant Contact, and didn't want to switch over to another way of handling lists.

Basically, use this if you actually need Constant Contact, otherwise, if you can choose either, use simplenews, it is in fact "better" as it handles RSS, is more configurable per-list, and is directly integrated with Drupal users, etc, whereas this just adds username and uid to Custom fields of the contact, and remembers who signed up.