Raspberry Pi vs Arduino ADK (with phone tethering)
A pi is indeed, not a wonderful solution for motor control - as an embedded linux device it is not ideal for realtime control, and as one which depends on an SD card for a root filesystem it is notoriously fragile in state especially where unexpected power loss may be an issue.  Being based on a set-top-box rather than mobile SoC, it also lacks most of the power control features one would expect for a battery powered system.

But doing USB host with an Arduino is not a great idea either - the host chip ends up costing more than your main microcontroller.  

If you want to keep the ADK idea with the external device as the host, look at something like a STM32F4 or KL25Z as an inexpensive embedded USB host (the STM Nucleo boards are in the $11 range, though the USB signals only exist on pin headers rather than a USB connector)

Another option is to turn things around and use the phone as a host, with the embedded processor as a device.  This is in a way more sensible, and can put you back in Arduino territory, however it does mean that you either have to come up with a way to power the phone while doing this, or end up running down its battery.

For simple motor control, running modulated signals through the headphone jack is worth looking at as well.

(I'm aware this is an old question, but there's nothing particularly dated about it.  Maybe with an answer we can retire it so that it doesn't bubble up again in the automatic churn).