Also on the www.brianmadden.com website is a very recent presentation from BriForum in Amsterdam last October where Ron Oglesby presents a head-to-head comparison between VMware ESX and Citrix XenSource.

Watch the video here: http://www.brianmadden.com/blog/GabeKnuth/Xen-vs-ESX-Head-to-Head-video

A couple of interesting conclusions out of this presentation. First, taken from one of Ron's conclusion slides, "While the XenSource Enterprise and ESX VMs are fairly close one on one, and processor utilization is pretty close, Disk I/O lacks in XenSource Enterprise. And once multiple VMs are running the VMware scheduler seems to be more effective."

Also useful is a run down on the testing from the comments:

1) Regarding CPU (and I think memory usage) and # of possible sessions, ESX 3 and XS 3 are quite similar, with ESX having perhaps a 3-5% lead in this area at most. Graphs etc. are quite similar.

2) Regarding disk I/O, IOPS and similar topics, ESX 3 is about 50% better than XS 3, so the question is whether one is willing and able to accept lower disk/data throughput to save money on licensing and put that money toward storage, which is the most expensive aspect of virtualization (that's why the $45-85 per writeable gig).

I think the primary update and more information that most people REALLY want is to know whether in XS 4.1 Citrix has closed the gap to say, 20-25% slower than ESX 3/3.5?? This much improvement would definitely make XS worthwhile for the majority of SMBs, since this would be "good enough."

The big question Ron asks in this presenation is, "Are these features worth the cost of ESX?" In my opinion, the best answer is "it depends". For lower-end environments that don't plan on extremely high resource usage or require true high availability, probably not. But for those environments that truly need those capabilities, yes. You need to determine internally what your needs and requirements truly are.