XenServer 5.0 has not been released for that long, but already a minor update has occurred. XenServer 5.5 is out now.
While largely I think XenServer 5.5.0 is a release to address some general bugs it also adds improved support.
The features added as part of the update are:

Improved Backup and snapshot support
Active Directory Integration
Workload balancing
Improved operating [...]

A few months ago I reviewed the performance and use of VirtualBox. A new version has just been released.
Version 2.1.0 is a major release and adds a whole new range of capabilities including:

Better hardware virtualization support on the Mac OS X operating systems

Better hardware virtualization support for Intel and AMD

full VMDK support

A better NAT engine

A [...]