It really depends on what you are looking for out of the product and what it will be used for.

As an example, Xen allows you to use different kernels on a per VPS level if you wish.

OpenVZ is pretty close to Virtuozzo as they are related, the pluses to Virtuozzo are the templating system and the control panel which I have not found any equal to.

The downside to Virtuozzo, it isn't cheap, but we expect it will pay for itself...

-Robert