JBoss Fuse is a open source (+ commercial support) variant of ServiceMix that adds the Fuse Fabric technology over the base ServiceMix for distributed management of large clusters of ESBs. In practice this means a central place (Fuse Management Console) from which you can manage the installation of your software across a cluster. Fabric also adds a runtime registry that lets your services advertise their availability and be accessed by other services in that cluster without hard-configuring locations.
Both can run ActiveMQ internally, as it is merely a bundle that runs in the underlying Karaf container.
The idea of subscriptions from Red Hat is that if you want to (there's no obligation) you can pay to have production support of your installation (someone to pick up the phone to if things go wrong), or developer support (help with building your apps to run on the platform).
Whether or not you want to run JBoss Fuse or ServiceMix depends on whether you feel you might benefit from the Fabric technology. There are companies out there that provide ServiceMix support.
For full disclosure: I used to work for FuseSource/Red Hat and now consult independently in the technology.