the difference in growth and the amount of time is really close. There was a post in here that demostrated the growth of plants on 24/0, 20/4, 18/6. The results are as follows or as close to measurements that i remember. The root growth for the plant that was on 24/0 had about 28cm growth, the plant that was on 18/6 got about 39cm growth, and the 20/4 was right in the middle with about 33cm growth. So it really boils down to just a couple of things, what is your pref in growth, what is your electric bill. Other than that the difference in 24/0 and 18/6 is really not all that much.