Regular ADK board vs Uno+USB Host Shield
The main advantage of the Arduino ADK is that, being purpose-built for Android accessory development, many libraries etc. are available for it. Google also supports it as the official accessory development platform for Android, but that just means you would need to find unofficial guides to help you (see below). Other that inbuilt USB and a  slightly more advanced processor, there would be no other advantage to using it over an uno with a USB shield.

If you want help on letting started with the ADK on an uno, this tutorial is pretty good: http://www.jayway.com/?p=9419/