Should show both, just as analog switches do. You want it to clearly unmistakably show the current state, but also the state to which it will change.
Apple has really nailed that with its sideways slider toggles. You can clearly see where the toggle is currently, what it will change to, and whether the current setting has made some feature active (blue background inside the slider) or inactive (grey background inside the slider).
Apple has really nailed that with its sideways slider toggles. You can clearly see where the toggle is currently, what it will change to, and whether the current setting has made some feature active (blue background inside the slider) or inactive (grey background inside the slider).