You can use either add-on or both side by side.
One example of using side by side would be if you wanted to not auto-detect and force iPhones to a mobile style, but still wanted to give them the option of browsing. You could add iPhones to the exclusion list in the
auto-detect mobile device hack and configure this addon so that iPhone users could still browse via mobile.domainname.com if they found the default style too slow or bandwidth hungry.