ajax AND codeigniter (in Controller OR in library)

[eluser]Unknown[/eluser]
Hi :-)

Excuse me for bad English (I use google translator)

I read many lessons for ajax AND codeigniter ....but I wonder what is better:

=================================================================

application/controllers/ajax.php
=================================================================
or
=================================================================

application/controllers/some_controller.php

Code:

class Some_controller extends CI_Controller {

    function index(){
            ....
    }

    function ajax(){
            ....
    }    

}

=================================================================
or
=================================================================

application/libraries/ajax.php

=================================================================

[eluser]CodeIgniteMe[/eluser]
What are you really asking? What do you need to achieve?