I don't understand why you want to use RS-422 line drivers ?
Using the digital interface takes longer to implement, but there are a lot more possibilities with the commands. Requesting the status for example.
So my advice is to use a digital interface, with the most common and most standard protocol and components.
The best option is to use RS-232. It is a common interface, and you can use a PC to test the servo driver.
The Arduino is very well capable to control a device with a serial port. But so are many other microcontroller boards and computers and tablets and terminals.
You could use an Arduino board with two serial ports, like the Arduino Leonardo or the Arduino Mega 2560.
One serial port is used to control the servo driver via RS-232 (with an adapter), the other serial port can be used to control the Arduino and to show the status on the computer via the usb bus.
If you have already an Arduino without extra serial ports, the SoftwareSerial library can turn any pin into a serial port. http://arduino.cc/en/Reference/SoftwareSerial
You need such an RS-232 to TTL adapter, as I mentioned before.