*I* did the experiment, myself, to settle the matter once and for all, at least as far as I am concerned. Here's how I did it. First, I bought two identical cabinets from WalMart. I added identical fans and lights to them (4" axial fans, 125W blue CFL in each cabinet, that's 125 real watts, not equivalent, big ol' bulbs). I put clones from the same mother in each cabinet. I kept the temps at 70F ambient in the room containing both of the cabinets, around 77F inside the cabinets. I fed them the same nutes (Fox Farm Grow Big), starting the nutes on the same day. After four weeks, the plants running 24/0 were noticeably larger and more developed than the 18/6 plants. I don't know whether the 18/6 plants had bigger *roots* or not, as I was just concerned with which lighting regime produced bigger plants in the shortest amount of time.

That said, the 18/6 plants did not end up, after flowering, all *that* much smaller than the 24/0 plants. They were smaller, but not hugely so. The six 24/0 plants probably produced about an extra two ounces (total, for all six plants) over the 18/6 plants. Still, a couple of ounces is a couple of ounces, and I'd rather have them than not.