My Understanding is that Geoserver can be run in the default Jetty Environment or in Tomcat on Windows Server 2008 R2
GeoWebCache uses Tomcat as its container in its installation instructions.
Which is better for Geoserver, Tomcat or Jetty and why?
UPDATE (Thanks @BradHards)
Application is stand alone Geoserver talking via Private LAN to a Standalone Postgres/PostGIS Server delivering up to 4 WMS tiled layers to the internet
Up-time needs to be 99%
Load is undetermined at this time. Deployment of additional servers is expected when load reaches a predetermined limit, which we are trying to assess.
Environment: Windows Server 2008 (32 Bit), Public Facing. Currently running Geoserver on Jetty
Updates: only when they will provide performance benefits or additional features that will improve the WMS Layers delivered.