Facelets is just a library of jsf tags that do UI layer stuff. JSP is servlet code that may or may not be used to do UI layer stuff.
Facelets main benefit is templating. Think tiles or tapestry, or a clever text parser you could write yourself if you wanted to spend the effort.
The main attraction to xhtml is that they can be validated. I think that is more secondary to the facelets people wanting to eliminate the JSP stack.