Well, the biggest difference is in the configuration interface. With Sail, the configuration is done through pages in the server-manager whereas in trixbox, there's a php front end (called freepbx). I've also integrate it in the server-manager. Another difference is that trixbox comes with many others applications like meetme (Web GUI for managing conferences), maint (phpMyadmin, phpConfigEdit, EndPoints manager etc...), xpl, a2billing, sugarCRM (or vtiger CRM) etc....
With smeserver-trixbox, you can choose or not to install all the others stuff