I think the XML solution would work better because it would allow me to simplify the new client registration process, while putting the smallest footprint on their server (most times a virtual hosted, or even free account).
From what I've read of the WDDX solution, the clients servers could use CFM,ASP,PHP,Perl or even JavaScript.
Hell, if a client is using a squatty server, then they could have a js interface that just displays the values, while still allowing them to pass form data (queries) back and forth.
The interactivity level is what's keeping me looking at XML and WDDX.
The only problem I've been faced with so-far is I'm comfortable w/ PHP, but MY vhost doesn't have WDDX compiled (what luck).