Well said Bonsaiguy.
I just want to add that marijuana in its vegetation stage has no requirement for rest in order to develop. So, no dark period is needed. The longer you have the lights on the greater growth in less days.
If you plan to grow out for a longer time though, 16 cycle is better than 24 since extended periods of 24 hr light can hinder the ability to accurately control the initiation of flower. You may get a spontaneous cycle change with power interruptions, or you may have difficulty convincing it to start flowering when you want it to.