[eluser]PhilTem[/eluser]
If you just want to have modular separation, you are fine with extending CI_Controller. If you want to call other modules' controllers from within your current controller, both controllers need to extend MX_Controller.
If you get an error extending MX_Controller make sure you are including the MX_Controller file:
Code:
include(APPPATH . 'third_party/MX/Controller.php');