i think 24/0 is more effective but if you are running a couple 1000 watters my vote is 18/6 to save on ye olde wallet since if i were to veg for a month / 31 days a 1000 watt hps at 11 cents per kwh costs 81.84 to run 24/0 where as it only costs 61.38 to run 18/6 saving me 245.52 over the course of a year per 1000 watter so if ya run 2 1000 watt hps you will save around 500 a year and im pretty sure i could find something to do with the 500 to improve my op so that i would never notice the loss in production
then theres the auto flowering strains that they reccomend 20/4 through out the life of the grow of which i have bo actual data