I'd go with the 18/6. On my first two plants, I went 24/0. I transplanted them at 3 weeks and they had good roots. Then I found out those were males. After doing some more reading, I grew a couple more (same strain) at 18/6 and transplanted those at 3 weeks. The roots were at least twice as dense. Luckily those both turned out to be females. Someone else around here posted some article about how dense root growth is more important for yield than denser foliage. So the 18/6 grew more roots for me, and I'm sticking with it. I don't remember there being much difference in how fast the plant grew, either. And a dark period may influence more plants to be females.