The compelling case for Win2k over XP would be that the hardware requirements are more modest.
This is less an issue that three years ago when XP was introduced, as the overall cost of a workstation including RAM, disk space and video requirements has essentially disappeared as a new purchase question.
One odd licensing issue that may help: If you purchase XP you are then permitted under most licenses to "downgrade" to Windows 2000. For a recent site that was reluctant to move to XP, the new licenses were XP and the installations were Windows 2000 using this license provision. This may help you in planning a transitional period. If you purchase Windows 2000, you would then in the future need to purchase Windows XP. If you purchase XP as new licenses for new workstations to replace older equipment, you are entitled to downgrade.
Downgrade rights allow users to install and use a prior version of an operating system than purchased pre-installed on a new computer. Downgrade rights are useful to customers who have application, infrastructure or other reasons for not wanting the current version of the operating system. Traditionally these rights have only been available to customers with Microsoft Volume Licensing Agreements (e.g., Select or Enterprise agreements). Microsoft Volume Licensing Agreements (VLA) are multi-year contracts governing pricing and usage of various Microsoft titles, including operating systems. However, with Windows XP Professional, Microsoft granted these rights to ALL Windows XP Professional users in the XP Professional End User License Agreement (EULA). The Windows XP Professional EULA extends downgrade rights independent of other licensing agreements; however it assumes that the user has purchased prior versions of Windows and has both the license and operating system CD.
