The only obvious difference would be the time it takes to start flowering. when lights are on the plant it is focused on absorbing as much as it can for growth while when the light is off it is focused on developing roots. So the 12/12 will generally develop a fatter root system compared to the one blasted with light all the time and the result is a better root system, allowing the ability to absorb more nutrients for growth when the light is on. In a growth race I believe that the plants that recieve a nice break do achieve better growth. Just like carokann's experience I have seen plants grow like a mofo in the night hours, seemingly far more rapid then when they had the light on them. In the end it would all depend on the time you want to place into the plant and when you would like to harvest.