Hi Emily,
You can think of Enterprise Sites as a subset of the capabilities that you get with ArcGIS Hub. Sites only provides the ability to build tailored websites and subpages and is more or less equivalent with ArcGIS Open Data.
You should use ArcGIS Enterprise Sites if you are using ArcGIS Enterprise, and choose Open Data (via Hub) if you are using ArcGIS Online.
Both could replace a traditional website depending on what kind of demands your organization has, meaning you have to work within what Sites/Hub allows you to do (but both are pretty flexible) and make sure that you have covered your bases when it comes to providing access to your organization (think about things like where/how you deploy, how much load you expect, making sure people have ArcGIS identities as needed, etc.)