I do 24-0 and I have plenty of roots when I transplant...sort-of. I still have problems with spin-out, but as far as seedlings and cloning, they above all need to root, and they do even under 24-0. Would the roots be "BETTER" under 18-6? I don't know...I DO know that theres room there to grow them...the roots collect along the bottom and sides of the pot, but hardly any roots IN the soil itself....
I'm not changing my light schedule but I will get some Griffins spinout, or the like and see if it makes a difference.
A suggestion if I may....could one of you 18-6 growers take one of your plants(male of course), and saw through the soil vertically, right beside the stalk, to the bottom, exposing the rootball and take a picture and post it for comparison to one done with 24-0 schedule? I would but am not on that schedule. This may help answer the delema.