I'm still on my first grow, but from my limited experience, I'm going to say 18/6 provides better growth on top of saving power, keeping it cool, etc. I started on 24/0, and the plants were growing pretty fast, but then I switched to 18/6 and they grew even faster. I don't know for sure that they were growing faster because of that, or any number of other factors, but about 75% of the growth seems to happen when the lights are off. This is true for flower as well as veg. I think the darkness actually triggers them from gather to grow mode, and without it, they just stay in gather mode.