I’ll show you an example to better compare the main differences between the two main models: the first will charge you for quantity of emails sent, and other that will charge you a fee for the amount of registered users.

Example: I have to send 4 newsletters a month to my 10,000 users listing.

Model 1: I have to pay for 40,000 sent emails.
Model 2: I’ll pay for the 10,000 users, on despite of the amount of newsletters I send.

With the Model 2, I can theoretically send a new broadcast every day and I’ll still pay a flat fee for my 10,000 users only… do your math and calculate how much will it cost with the first option… is it clear which is the method I selected?

Of course it’s not that simple; you must consider other details among value for money like: data safety, deliverability, technology, reporting and a customer support. I recommend you taking a look at GetResponse.