The most common uses for a cluster in VMware are to take advantage of HA and DRS. As a best practice we must try our best not to have hardware with different specifications in the same cluster. If you add hosts with different specs into the same cluster you end up with an unbalanced cluster. When you have an unbalanced cluster the Admission Control Policy calculations will not be what you might expect. When Admission Control is enabled the worse case scenario is taken into account. Which means, that when the calculation is made for available slots, the largest host is essentially left out of the equation. This is in the case of setting your host failures to a single host. Due to this, the best way to make sure you have setup a good cluster is to deploy machines with the same amount of resources for both memory and CPU.
Hope this help..
~Nitesh