Comet or Ajax ?

Which one you should choose ? Ajax or Comet ?
Comet changes the traditional model by using a long-polling HTTP request in the background that allows the server to push data to the browser without requiring additional requests. Essentially this is like AJAX, except in the opposite direction.
While the AJAX model increases the richness of the User Experience for a single client at a time, Comet can do the same for multiple users.