Well, there's this little blurb in the faq[0], not sure if that helps you any.

[0]http://www.polymer-project.org/resources/faq.html#xtags

One of the biggest differences is that X-Tags doesn't have templating or data-binding, or some of the sugar for reacting to DOM changes. With X-Tags you either manipulate the DOM directly, or bring in another library.