A USB device can negotiate for 100-500mA of current - but don't need to draw the negotiated current.

But my vote is for the headphones alternative consuming less. It's a quite small amplifier that is used.

The main question is the consumption of the USB hardware and how much "sound card" that needs to be enabled when sending out the sound digitally or through the analogue headphone connector. Even a small difference in CPU load will also matter greatly.