I had it explained to me by someone who actually tried both ways with cannabis and these were his results. Dont really remember word for word how it went, but it was something like it doesnt really matter either way. On a 24 hour schedule your plants will actually grow slower, but since they have an extra 6 hours of light compared to an 18/6 schedule in the long run your plants will grow bigger. Something like 25% more light, but only 15% more growth. So if your in a big f'in hurry for some reason, use 24/0, if not, 18/6 is just fine. In my opinion my plants look happier under 18 hours light than 24, i have tried both ways also. I also think, just my opinion, but i think its more efficient to use 18/6. Maximum plant growth for the amount of light, im all about efficiency in the growroom.