From a technical point of view, working with the API is more convenient, since it "hides everything unnecessary under the hood" of the system that implements working with API methods. While using SCI, you must create an unnecessary step every time - generate and submit a form.
Moreover, SCI can only be used to accept payments on the merchant's website by generating a form. And the API allows you to use methods of accepting payment, conversion, out payment, and much more. Thus, the API method automates the work and does not require manual intervention.
It is worth noting that in order to work with SCI, a merchant just needs to add a web form to his site, while to work with the API, you need to configure a json-rpc client.
API is a multifunctional interface for all possible payment transactions. A complete list of API capabilities can be found in the technical documentation or checked with our support team.