Mailgun vs SendBird

For transactional emails, notifications, reminders, etc, I want to make it so writers/designers can set up the emails and maintain them, and then dynamically insert fields, that I then replace when actually sending the mail from code.

I think the ability to use a basic layout template across individual email templates would make things a lot easier (think header, footer, standard typography, etc).

What is best for this? Why would you prefer Mailgun, SendGrid, Mandrill or something else?

While building our authentication system, we originally picked Mailgun. However, emails took minutes to arrive and some of them didn't get delivered - or got delivered to spam.

We started looking for a new provider, and settled on Postmark. We love that they track time-to-inbox, it makes me feel they really care about going above and beyond to provide a good service.

The problem I have is we are using mailchimp and not happy. We want to switch to mailster and are curious the easiest and best way to send emails with mailster.

The most important factors for me are ease of use, tracking.

I have only had experience with mailchimp, constant concact and streamsend. So ease of use is important.