What way of controlling volume is more battery efficient? I have a headset with it's own volume control, and I can also control the volume of the music through the android's volume control keys.
But which way of controlling is more battery efficient?  


Have my android device on full volume and control it with my headset's control.
Have my headset on full volume and control it with my android device.
None of the answers above. (if so, then which?)


EDIT: I'm not talking about an electronic controlled headset, I'm talking about a headset controlled by a dial resistance.
Conclusion first: It doesn't really matter as this operation needs almost no power anyway for both ways.

Changing the volume alone has only a minimal affect on the battery given the screen stays off.

Reason: If you listen to music there's already some processing going on and increasing/decreasing volume levels has a very low processing demand (almost none). It's more of a side-note event for Android.

The volume buttons on the side of your device trigger some event and so do the buttons attached to the string of your headset. I'd say it's equal or at least very close to equal.

You can have a look over at George Smart's HTC headphone analysis and see that the L channel is not shorted against the common ground. Instead some coded resistor values is used to pull the common control line (also used for the microphone) which probably has a high impedance.