I've seen this debated a lot on different forums. Try searching. The general opinions I've read veered towards favoring 18/6, with 24/0 still favorable and recommended by a minority. While some others said 20/4 gave them the best results. Apparently your plants will grow taller with a dark period but as far as I know it's not proven if they actually grow more mass/foliage. Though roots apparently grow more with a dark period, and its been suggested that you'll get better root development with 18/6 or 20/4 than 24/0.
A few minority voices said they had plants preflowering after being switched from 24/0 to 18/6.
Most growers seemed to accept that both 18/6 and 24/0 work well.
I'd try both and see what you prefer.
Personally what I'd try is either 20/4 or 24/0. The main reason I'd choose 24/0 is simply to avoid using crappy unreliable fire hazard wall timers, which I have an irrational fear of.