Both configurations using XML files and annotations are equivalent. In my opinion, it's just a matter of feeling and learning-curve.
The major benefit using annotations is that you can a priori get rid of the struts.xml file (yeah, with all the frameworks using XML declarative architecture, removing one of them can alleviate the classpath content). But from my little experience, you can't consider not using the struts.xml file for large projects. Also, as you mentionned, using annotations, you can see the power of inheriting intelligent defaults provided by the framework. Therefore, some annotaions are not mandatory, struts 2 will automatically use a default configuration(it's called zero configuration, or convention over configuration).
The major benefit using XML file is that you have a centralized way to manage your application architectural components, even if you can modulirize your components declaration in multiple XML file (which are referenced in the struts.xml file). I guess using XML file alleviate the learning-curve. XML file also allows you to inherit intelligent defaults.