In the Web 8 CD installation instructions when installing the micro services for an XPM enabled preview site it suggests you to install a "Session-enabled Content Service" registered as "ContentServiceCapability", and a "Session Preview Web service" registered as "PreviewWebServiceCapability". However they use the same "preview\service\" pre-configured role source files for both services, with slight differences between configuration.
In the Quick install guide only the "Session-enabled Content Service" is created and configured for both the "ContentServiceCapability" and "PreviewWebServiceCapability".
What is the difference between the two? In what scenario would I choose the first and second configurations?