The main difference (IMO) is you can use your model objects (i.e. Hibernate POJOs) in your view with WebWork. With Struts you have to use ActionForms. Struts has better client-side validation support, but WebWork's validation is more full-featured in its expression syntax.
If you want to compare the code needed to develop in Struts vs. WebWork - you could take a look at AppFuse, which has options for using Struts or WebWork (or Spring MVC) as your web framework.
I think in a way Struts is intrusive, requiring you to code something very according to their "spec". And also, I ahve read a series about MVC taken completely wrong by Struts (but again I cannot remember the links)