I won't go shouting ones better than the other.
However for Elastix, (especially not requiring disk speed), they are pretty much suitable for the job.
I have to admit, I have not had a look at "templates", but a base install on either will work, and then a bit of "tuning", more so to make sure that they have the resources they need, and likewise what they don't need...is all that is required, and this might be a bit of performance monitoring.
VMWARE, you will find more third party apps, but then XEN with its Central Management puts it out in front as well. I am just building a new Xen box in the lab at the moment, but just on the back burner as I get other things out of the way....
There is almost no knowledge needed for both, plenty of docs on the web and if you follow the guides for installing Redhat, you will find most applies to the Elastix ISO being Centos....
Just a final word, when I did my first install on VMServer 2.0, I used no instructions....and subsequently all the versions since then, pretty well straight forward...XEN, just a little more thinking, but not that much more...
Regards