Unplugging iPad/iPhone Charger: Which end to unplug first?
There really isn't any difference, there is no chance of anything bad happening regardless, although if you were particularly worried about this sort of thing, then frankly you would be better to turn the charger unit off at the wall socket first, after which it is utterly irrelevant (assuming you are using a wall charger).  If you are charging from a PC with no option to turn off the USB port as such, then you might have a preference depending on if you leave the cable in or not etc, but it's just that; a preference.