A. OpenID Connect Web Server Flow. Determine if the service provider is secure enough to store the client secret on.
B. Embedded Login. Identify what level of UI customization will be required to make it match the service providers look and feel.
C. Salesforce REST apis. Ensure that Secure Sockets Layer (SSL) connection for the integration is used.
D. Embedded Login. Consider whether or not it relies on third party cookies which can cause browser compatibility issues.