Extending battery life is one of the main reasons I run my tail lights in flashing mode, on the rear rack and back of the helmet. I usually run the flashers day and night and on long rides they need to last several hours. Battery life is equal to visibility in my preference for flashers over steady red rear lights. And my spare red LED is a slow flasher only.

For the front light, it depends on the situation. I prefer flashing white in city traffic since there's usually enough ambient light to see the road at night. But I'll switch to steady when I need to see the road. I'm convinced from seeing drivers notice me in heavy traffic that the flashing white light gets their attention and prompts them to pause before leaping in front of me from intersections and parking lots. I keep the helmet mounted light tipped downward a bit so it's not in anyone's eyes unless I tilt my head back and look directly at them - which is also very effective as an attention grabber.