if you use flash and photoshop that are not replaceable with microsoft solutions maybe it's more convenient to use only adobe software.
dreamweaver is also leaps ahead expression web designer, especially in standards support.
I am also interested in the new document type one of the Expressions (can't remember which) comes with (ala Acrobat).
XPS? it will take years to become mainstream because only Vista can read it without installing NETFX 3.
I recently went to a Microsoft conference at my company, I was given a brief expressions demo there, from my understanding, Expressions Suite is remarkably similar to the Adobe CS Suite. Because of that limited knowledge, I posted here for a better understanding
prior to investing one way or the other.
I'd try the trial versions of both before deciding. but as long as I could see the Adobe suite is a far more advanced web designing solution.