I don't have any scientific data, but from personal experience I have found the Rd-pod to be more accurate than the watch alone. I think it's easier for a sensor to get that type of data when it is fixed in the middle of your back while running vs your watch which is flailing about at the end of your arm.
Also, the Rd-pod gives you left/right balance which the watch can't do by itself since it is worn on one side of your body and not in the middle.
