I don't have enough knowledge of the Spring MVC so I'm not going to comment on it, but compared to JSF, Struts has a simpler life cycle and I suspect it's faster to build Struts apps than the JSF equivalents.
One 'killer' feature of Struts 2 I would say is the plug-in. Thanks to this extension mechanism, you can build and use 'components' easily. Third party components are in abundance and can be found here:
http://cwiki.apache.org/S2PLUGINS/home.html In my book, plug-ins are discussed in Chapter 23.