Both are quite easy to deploy and Xen performs faster than ESX, but I never want to even think about running non-clustered virtualization solution. In this aspect does Vmware come in front. My bosses argued that the solution I select must have some level of official support and my free time is too valuable to spend saving 40 to 60 production virtual servers from crashing due to package update. I went to a Citrix presentation, where they showed us features of their Enterprise solutions. I must say that it worked flawlessly and I liked it, altough I am OSS fan. But final cost of Citrix solution is almost the same as Vmwares, if not higher. I was also considering using SLES10 Xen and Zenworks Datacenter management tools, which provide a high degree of availability, but in the end, when technical and financial aspects of every possible solution were compared, Vmware was clearly the solution we had to accept to achieve our goals. For next three years, we're commited to Vmware, but closely watching Xen. I hope I'll get to run it in a datacenter one day :) THe sooner the better.