I am aware this is an old thread and I am bumping it, but it comes up in searching “extension routing” vs “class of service” (along with this thread that pretty much says CoS has more features), and I want to answer the question in case it comes up in anyone else’s searches. The Extension Routing commercial module does NOT show up in a separate menu item, just go to the extension you want to route and there is a section under the Other tab. Please see the wiki for details:
https://wiki.freepbx.org/display/FPG/Extension+Routing-Admin+Guide