The CA Directory dxdumpdb tool will export all the inherent data structure with data from the LDAP instance, including objects that are strictly related to CA Directory in a format that other tools from CA Directory will be able to use.
The XPSExport tool on its side will only export the SiteMinder objects. It won't export the data structure. Remember that by constructing the Policy Store, you had to run the command XPSDDInstall. This tool is to create the data structure in the CA Directory instance.
So XPSExport is Siteminder application related and dxdumpdb is CA Directory service related.