Difference between Esri ArcGIS Server Feature Service Reader/Writer.and Esri ArcGIS Portal Feature Service Reader/Writer

A simple answer summary would be:

1. ArcGIS Server reader/write is looking for a REST URL of the web service. It offers more basic authentication and is often used in open data portals where you need the URL and often need to use the GetCapabilities to understand what the service has enabled.
2. The ArcGIS Portal Feature Service will utilise your Server Enterprise organization and authenticate typically using oAuth protocol via a web connection. It requires some setup from your ArcGIS administrator, where you pass a client and secret into FME web connections under manage services. When you authorise, you use your account to interact with the organisation, meaning you only access content that is shared via groups. The advantages are many, particularly around discovering all the feature services in your organisation via parameters within the reader/writer.

I don’t think much fundamental functionality is different back in 2018, but an upgrade to a recent version would have a number of potential bugs removed for the Portal. I appreciate you need to consider ersi/fme compatibility