Most people go for the 24 hour constant but i'm not 100% sure that 18/6 off makes all that differance for some reason some folks don't seem to think plants grow at night .i have tested this. I put one set on 24 and 1 room on 18/6 there was'nt that much differance and i also made notes of size of buds when lights came on and when they were off just b4 lights came on and there was growth during the lights off period. Plus i also think that it helps the plants to cool off a little and it also helps your gear.it gives you lighting equipment a little break time.im not dissputeing that the plants been on 24 hour will grow faster than plants on 18/6 but i think the plants defanatly benefit from the break in the lihts.but the choice is yours luke and may the force be with you.