VMWare have bought me lunch, Xen haven't. Therefore, I like VMWare
It's an odd question. What are you trying to do, what's your budget, what's your current server estate - the answers to these might cause you to prefer one over the other. If you want the management capability with the power saving stuff and high availablity thingies of VMware, then that works quite well in my experience. If you want something that works for a bit less money, and you have the hardware to support the 64-extensions that I believe Xen needs, then Xen might be the better option.