The choice on whether to use SMTP or API is completely different and dependent on your needs as well as the capability of the platform you are using to send this emails.

SMTP

API

Easy to connect and can connect to any system

Flexibility in sendings

Doesn’t involve any development from your side

Greater security

Better on a larger scale

A cloud based SMTP service might be as effective as a HTTP API and yet an API could offer a bit more flexibility in the level of customizations as well as faster times and relatively low chance of failure due to verification processes.