I think the "hit" on battery life caused by running in the higher screen resolutions would depend on how often the screen needs to be redrawn. Showing video or running a game, for example, might require the processors to work a lot harder rendering the increased resolution images than they might have to work rendering lower resolution versions of the same images.
So for normal web surfing, texting, or even viewing still images, we might not see much increase in battery usage when running in the highest resolution versus lower resolutions.
Of course brightness will always affect battery use, regardless of the resolution setting.