oVirt node is a dedicated hypervisor imgbased installer distro for oVirt that handles upgrades all at once with a rollback feature, whereas an Enterprise Linux Host would do the same thing but you would partition your disks however you saw fit and manually install RPMs to get the same functionality as a node-ng host.  No need to have both type of hypervisor hosts.  The engine is sometimes a VM itself which provides the Web GUI and databases needed to manage a cluster of virtualization hosts and VMs therein.