Sorry I haven't been monitoring this thread.
Improv, I too would have directed you to Xara's website as you will find a much more detailed answer to your question. In fact, I still recommend it. However in the interest of answering your (ever so humbly asked :D) question and wrapping this one up, here are just a few examples of what XXP5 has that XWD does not.
Bevel tool
Contour tool
Blend tool
Mold tool
Live Effects tool
Extrude tool
Several extra file type export options
The ability to make stretchy buttons and text panels from scratch (as opposed to modifying one that exists in the designs gallery)
The list goes on, but you will need to go to the sites provided by sledger to learn what all of them are.
Eric