AJAX Auto Completeextender

Question

As per my knowledge, we can use AJAX AutoCompleteextender via two ways either using 'Webservice' or by defining Service method in 'ASPX or ASCX' file.
 
I just wanted to know that is there any advantage in using webservice for AutoCompleteextender instead of writing Service method in ASPX or ASCX file?
 
|Thanks|