You are probably aware of the main problem in using Usb, which is having one of the devices operating as a Usb host. That's basically what the Arduino ADKs are all about. If the Nexus can provide this functionality you can probably do without the ADK. Both ways, you will then have to work on serial communication. A nice feature of Usb hosts is that they can also provide power to connected devices, so if you choose the ADK as the host it will charge your Nexus
Not exactly your requirement, but you may probably solve the problem through a powered Usb hub.