Well sort of an answer anyway...
The Technical Marketing team (isn't that a oxymoron?) from Citrix and Microsoft recently got together and published a 14 page PDF (what no XPS?) where they've compared the technical features of a Server 2003 TS, Server 2008 TS, and Citrix implementation. While I personally think that both teams have been biased in certain areas of the document, it is a good general comparison of the features between the two products.
Click here to read the Citrix blog entry that links to the pdf.