XenCloudPlatform - is actually Citrix CloudPlatform (CCP). This may change based on the recent announcement about Accelerite (Persistent Systems) acquiring CCP.

CloudStack - is Apache CloudStack (ACS).

The primary difference is the availability of formal support for CCP from Citri (for break-fixes/technical support/consulting).

In terms of the product features and reliability, while CCP is based of ACS code base, there MAY be,

a. additional bug fixes in CCP that are not present in the currently released version of ACS

b. feature disparity; some features in ACS may not be supported in CCP. You may refer this doc/link (chapter 4) for more details. The reason for this disparity can be attributed to the fact that these features are relatively new and untested in a production environment so excluding these may result in better reliability in a production environment.