What's your pricing model? Why?

We provides our service at per month, with three tiers. I did some A/B testing to see if other pricing models but everything pointed to a monthly tier based.

Our pricing is based on the volume of data a user is intended to need. The more emails you need to build and store the more expensive plan you need. We don't hide product features behind the paywall as we believe that everyone has to have the same experience, no matter how thick their wallet is.

we're deciding between seat-based (number of users) and usage-based (storage/uploads) pricing

Freemium.