Well, if you can't really tell the difference between the two controllers, what does it really matter?

I have no idea about model differences, but you should be able to feel it in the joystick and shoulder buttons, and possibly the regular buttons (B particularly). On the old one, the joystick might be looser, the should buttons less clicky/
more sticky, and the b-button might feel mushy. Also look on the joystick for dirt/grime.
Other than that, whatev!