XDE is IBM's WSADD with a modeling perspective added by Rational. It's built on an older version of Eclipse (1.0 I think) so you don't get any of the Eclipse 2.0 features.
If you already have WSAD 4.03 then
you should be using the XDE for WSAD version.
What you are getting is the modeling features of Rose with the IDE features of Eclipse (at least IBM's version of Eclipse). IMHO Rational made his move to combat what was becoming a strong TogetherSoft TogetherJ presence in the market.
XDE gives you a little better GoF
Patterns support than Rose but I still favor Rose since it is primarily a modeling tool with some code generation capabilities.
The place to go for support is
www.rational.net also note that Rational is offering free web based training on the web site for XDE - I've went through, not great, but it'll get you, modeling and operational.
HTH
[ March 03, 2003: Message edited by: John Pritchard ]