there might be actual science to answer that q but here you will find 50/50 split guaranteed.
either or both will get you results,, i have never done 24/0,, btw,, not 24/7 (as compared to 18/6),, and would not do so for the simple reason of extra cost and heat
Plants do benefit from some dark time. You'll have good results with 24/0 but having said that, you're depriving the plants of that beneficial dark time while also increasing your power bill for no good reason. 18/6 is the better option.
When at home in my own garden I chose the 18/6. When I volunteered at a few commercial grow sites, everything in veg went 24 hours. There speed was a factor over plant “happiness.” They will grow with either.
I believe that I see better root production and more growth at night than at any other time. It makes sense that the plant uses the dark period to get things done and move things around within the plant that cant be accomplished while full photosynthesis, transpiration and energy gathering is going on. I have never done a side by side comparison, but it is gut feeling that more rapid growth is seen by plants that have at least a small resting period, and I run 18/6 in veg because of this.
I have also seen in my time lapse studies that the plants anticipate this rest period and droop a bit, letting the pressure in the xylem go down, presumably so that materials can freely move out of the leaf storehouses to the areas that need them, and then again in the morning, the plants predict the turning on of the lights and start up transpiration again, lifting all the leaves up to greet the sun. This phenomenon, the plants reacting in such a visible way to the light and dark periods, tells me that the plant needs these distinctly different situations, or why else would it be able to react to its daily cycle in such a regular and dramatic way?
I agree with nivek, lower cost and less heat.
I run my young one on 24/0 for the first few weeks to give them the extra light, then switch to 18/6 to fill them in.
I also run T 5's for those first few weeks of growth then switch to an SS 400 LED for the remainder of veg.