This is taken directly from the High Availability best practices guide;
The overall size of a cluster is another important factor to consider. Smaller-sized clusters require a larger relative percentage of the available cluster resources to be set aside as reserve capacity to handle failures adequately. For example, to ensure that a cluster of three nodes can tolerate a single host failure, about 33 percent of the cluster resources are reserved for failover. A 10-node cluster requires that only 10 percent be reserved. This is discussed in more detail in the Admission Control section of this document. In contrast, as cluster size increases so does the management complexity of the cluster. This complexity is associated with general configuration factors as well as ongoing management tasks such as troubleshooting. However, this increase in management complexity is overshadowed by the benefits a large cluster can provide. Features such as vSphere DRS and VMware vSphere Distributed Power Management™ (DPM) become very compelling with large clusters. In general, it is recommended that customers establish the largest clusters possible to reap the full benefits of these solutions.
My view would be to leave them separate unless you have a good reason to merge them - ie. a technical or business reason. This is will certainly keep the implementation simple and also make troubleshooting a little easier if the host configuration (including hardware) is identical for each host in the cluster. Besides the failover resources I can't think of a compelling reason to merge them.
Cheers,
Jon