Lumens (light output) per watt (power in) is much higher for LEDs than incandescent.
However LEDs can be very directional in their light output and the spectrum emitted can be an issue. So sometimes an incandescent bulb may be a better choice, at least at the moment. LEDs lights are under rapid development and color rendering (emitted light spectrum) is getting better all the time and new ways are being developed to make the light intensity more uniform so the number of applications where incandescents are better than LEDs is shrinking all the time.
Typical high power LEDs actually need about 3v DC to operate, so anything that is developed for a 12v system could, in theory, be applied to a 6v system. The issue there being lack of volume to induce a manufacturer to make a product.