i personally prefer 24/0, it works great for me, and i have great fem/male ratios. out of my last 20 plants from seed, 18 have been female (cool temps, low p in veg
)
but, the statement of 18/6 comes from a very experienced med grower; he and we have fought several times over this issue, and i deduce that it's simply a matter of prefference. both work, i like 24/0, he swears by 18/6. in fact, there are many books that echo his arguments. but then again, there are books that support 24/0 too...
so, that's where i stand. hope it helps your decision, as i seem to be contradicting myself