Essentially, the answer is If you have the rep to vote to close, vote to close. If you don't, flag. 

Note that the [vote down privileges][1] are granted at 125 reputation, and the [close questions][2] privilege is granted at 3,000 reputation. 


  [1]: http://askubuntu.com/privileges/vote-down
  [2]: http://askubuntu.com/privileges/close-questions