Usage based pricing or subscriptions? What's better?

I'm building cocomail.io

cocomail is an email automation platform for SaaS

Let’s you send transactional + marketing emails with an easy & simple api integration.

Unlike a lot of companies we don't plan on charging based on number of contacts but based on number of email you send per month.

But i'm also debating with my founder whether we should do subscriptions or usage based pricing?

One will only pay if they send emails or no charge..

what are pro's and cons of usage based pricing model?