XML versus Relational Database

Hello, 

 Although I am developing an application for securing XML data. This
morning I woke up and came out a general question: Is it really a better 
idear to store data in XML structure instead of relational table? As we
know to bulid a DOM tree is quite expensive for main memory...Any advice
and opinion are cherished...:-)
  

Best Regards
Wang Yue

    *------------------------------*
      Attitude makes the difference
    *------------------------------*