VMware is market leader for server virtualization solutions and offers with vSphere 5.x de facto the standard.
With the consolidation of the server farms massive cost saving can be achieved. Administrators also benefit from the centralized management - here VMwares vSphere especially differentiates with a high level of clarity and many helpful features.
Microsoft offers with Windows Server 2008 R2 a powerful server platform. The WS2008 R2 suits for SMB and large companies equally.
Important innovations in WS2008 R2 are improved power management, the powerful connections of branches as well as the integrated virtualization (Hyper-V). Hyper-V offers the possibility to create and administrate virtual machines directly under WS2008 R2 without additional software. So there is no familiarization to a new platform necessary for experienced Windows administrators.
WS2008 R2 is available in editions Standard and Enterprise for the Zealbox server.