Using SOAP Web Services or REST API Resources, if you want to deliver strategic digital assets for your organization, SOA aproach is an excellent option. The InterSystems IRIS supports like a charm the SOA principles with Contract First technique to model services aligned with the business, and create the services from the service contracts (Open API or WSDL).
The InterSystems IRIS allows you to reuse the services using productions or consuming SOAP or REST between services (I prefer first option), but this consume management, if you want to get governance, reuse and management, must be from endpoints in the IRIS API Manager or IRIS ESB Service Registry. It is, because they will provide to the organization security capabilities, a corporate digital asset library to maximize reuse, the service lifecycle/version control and other important features.
See: https://docs.intersystems.com/irislatest/csp/docbook/Doc.View.cls?KEY=PAGE_apimgr for REST and https://docs.intersystems.com/irislatest/csp/docbook/DocBook.UI.Page.cls?KEY=EESB_registry_admin for SOAP