Hmm... found a semi-satisfying answer to my question about the difference between YDS and X1 here http://labnol.blogspot.com/2006/06/x1-desktop-search-is-now-free-as-in.html
Joe Longtin who is Product Marketing Manager at X1 Technologies explains the difference between Yahoo Desktop Search and X1 Enterprise Client:
Yahoo! Desktop Search includes support for the Yahoo! Address book and their instant messenger. It’s excellent for personal use on one’s desktop and it will continue to appeal to the consumer of Yahoo! Services, at home or at work.
X1 Enterprise Client is intended for intensive business use in a dynamic environment, therefore it's highly configurable, extensible and - as one component of the X1 Enterprise Platform - scalable.
* Real-time indexing. Changes to files and deleted files are updated in the index automatically.
* Support for Outlook Calendar and Task items. Users can search against virtually anything in Outlook.
* Range searching means users just have to know a specific date/numeric range, not an exact date.
* Drag and drop search results into emails or onto your desktop.
* Support for Lotus Notes 7.0.
* The X1 Enterprise Client federates search results from both the desktop and the X1 Enterprise Server.
* Unicode support. Search multiple languages.
* Ability to search network folders and enterprise content repositories. Using the free Enterprise Deployment Kit, IT administrators retain central control and can "switch on" end user access to network locations. X1 maps to Windows ACLs.
* Group Policy support.
The Enterprise Client really appeals to IT professionals who are under pressure to make their "customers" more productive without compromising information security.
Given that I don't care about Yahoo Messenger and Address book indexing AND X1 offers real-time indexing and apparently greater security this is starting to look like a no brainer.