If you want to use Android with an Arduino and decided to use something like Mega anyway, I would recommend the Arduino Mega ADK. The main difference between the two is that the ADK has built-in USB host functionality, so no extra USB host shield is necessary to create a dock like you mentioned.
Something like this Android App based on Firmata might help you as well. It's primarily for BlueTooth interface, but USB interface is also available via a USB host shield or an Android ADK and a high enough version of Android.