Many points of overlap: repeating controls, client side validation, output fields, etc.
XForms is a finished spec.
XForms has multiple existing implementations in Flash, JavaScript, server side frameworks, and plug-ins.
XForms uses a model-view-controller architecture.
XForms can generate essentially arbitrary XML.
XForms separates content from presentation better.
XForms is based on XPath; Web Forms 2 on JavaScript (especially for output fields)
XForms validates based on schemas; Web Forms based on simple types.
XForms is much more complex.
XForms is less backwards compatible with existing browsers.
Web Forms is supported by Opera and (to a lesser extent) Apple, KDE, and Mozilla.
XForms is supported by IBM, third party vendors, and (to a lesser extent) Mozilla.