In the interest of saving on energy alone, 18/6 would be better obviously. However, this would be my choice anyway because I strongly believe plants need a dark period. Do you know of any place in the world where daylight lasts 24hrs. for more than a day? You are trying to replicate a natural light cycle indoors and 24hrs. of light is just too unnatural for me to endorse.
I hear you Roor, my girls went 24/0 for 45 days and were slow growing, however very strong. They took way too long to get to flowering stage in my opinion. But now that I grow only 12/12 from the start, it doesn't matter
IMO if your vegging with HID's use 18/6. Vegging with floros use 20/4. The extra light helps with the less intensity of the floros while still allowing a sleep period. It's been my method for years now with great results
I've been using 24/0 mostly. I think vegging at 24/0 is a bit slower as far as the plants growing in height, but the nodes seem to be much closer together. I love having the branches very tight. It makes for some killer baseball bat colas. IMO. I would suggest if you decide to go with a certain light regiment don't switch it up. For instance. If you start out 24/0 don't switch it up to 18/6 a month down the line during veg. I've read in the Grow Bible that this can cause your plants to Herm.