struts.xml and struts-config.xml

The core configuration file for the Struts framework is by default the struts.xml for Struts 2 and struts-config.xml for Struts 1.

They are both configuration files so that is the same, but they are different beasts because they refer to different versions of the Struts framework.

There are large differences between Struts 1 and 2 as you can see here.

The files have different structure and content so you can't really compare one with the other (except for their purpose).