Where VMware charges more than its competitors is in its portfolio of management tools that make the hypervisor more useful, Greene says. Live migration, for example, is one feature that must be purchased, according to DiDio.

Greene says VMware shifted its value from the hypervisor to the management and automation products because it expected competitors such as Microsoft to enter the market and offer a hypervisor that's free or almost free. (See related story, "VMware, Microsoft battle over virtualization management capabilities".)

Greene touted features such as memory over-commitment, which increases memory utilization by provisioning more memory than is actually available on a physical server.

"For example, the sum of the memory of all virtual machines running on a server with 8GB physical memory can be 16GB," VMware states on its Web site.

Microsoft offers some advantages beyond price, however, Microsoft System Center is capable of managing both Hyper-V and VMware virtual servers, Voce says. System Center also can manage both physical and virtual systems, another advantage over VMware, he says.

VMware manages only its own virtualization products, and "takes it for granted that everything [in a customer data center] is virtual," he says. "Beyond that you need to look to the system vendors to provide systems management."

Most people choose VMware because it was the "first in the pool" and has the best reputation among virtualization vendors, DiDio says. There's no reason to rip out VMware technology if you already have it, particularly if it's working well. New customers have a tougher choice, and may want to at least test out the free VMware Server before doing a full rollout with either VMware, Microsoft or the open source Xen hypervisor.

"Up until now, customers deployed VMware's hypervisor, particularly in the large organizations, because it was the only game in town, the best game in town," DiDio says. "That's changed now; there's a lot more competition."