There is a thread around here some place that does a side by side compare to 24/0, 20/4, 18/6. What they had found is that the 18/6 had the most root growth of 39cm while the 24/0 had a root growth of 24-28cm and 20/4 was in the middle. They all will work just find, so its a personal preference/electric consumption that you will need/want to discover for yourself.