This White Paper is already one year old, but still valid!

The Microsoft War on Cost team conducted research to determine the on-going operations and management costs for Hyper-V in comparison with VMware’s ESX and vSphere products. The research intended to answer two questions: which platform is less expensive in IT labor to operate on an on-going basis, and do the systems management products used to manage the environment impact the cost of managing either or both platforms. The War on Cost team used HANSA/GCR, a market research firm, to deliver a web-based survey to enterprises with 500 PCs or greater and collect the results.
The research results show that customers running Hyper-V spend 24% less on IT labor, on an ongoing basis, than customers using either ESX or vSphere.

http://download.microsoft.com/download/1/F/8/1F8BD4EF-31CC-4059-9A65-4A51B3B4BC98/Hyper-V-vs-VMware-ESX-and-vShpere-WP.pdf