I am thinking through a federated authentication approach requiring the use of a SAML 2 Identity Provider. The same Identity Provider infrastructure will be used for both the Sitecore admin and website authentication. It looks like the two options are...
Implement directly using ASP.NET Identity (OWIN Authentication middleware) introduced in Sitecore 9.0, widely referred to as "Federated Authentication" in the docs.
Use Identity Server as a federated gateway and create a custom plugin for the SAML 2 subprovider and deploy to the Identity Server Sitecore Host instance.
Implementing on top of the OWIN middleware looks to be more straightforward from an architecture and implementation perspective, any reason someone would caution against this route?