Headphones draw roughly 20 mW (generic run of the mill, can go as high as 2000 but not in N900). USB draw varies (generically speaking) from 100 mA@5V (500mW) to 500 mA (2500mW), depending on what the device requires/requests.

So, generically speaking, USB draws more. It is possible, however, as attila77 said, to have self-powered USB, in which case the data link draw would be even lower than headphones.

When playing audio, your biggest draw is the player software, decoder, audio layer, etc all keeping CPU up.

Also, if you have an external amplifier, low volume audio (down to line level, maybe with added electronics) would give you the longest play time. But then again, if you plan on carrying extra weight an external emergency pack will likely be more useful and not always-connected. Cheaper, too, most likely.