The one on eBay (from China) uses CP2102, which I personally like better than FTDI chip, but there are more FTDI documentation for communication with Arduino.
Just keep in mind, eBay one ships from China and it's free shipping (meaning it's probably is going to ship via their postal service) which may take up to a few months to arrive.
If you want to get it from the US here are couple of links:
CP2102 based adapter: http://www.robotsimple.com/USB_to_Serial_Adapter
FTDI based adapter:http://www.robotsimple.com/Arduino_shields_and_accessories/FTDI_Friend_plus_extras_v1.0