More light or longer daylight hours?

So, longer daylight hours and lower wattage or shorter daylight hours with higher wattage? Anyone have any experience with this? A 250 watt metal halide on for 16 hours a day or a 300 watt MH on for 14 hours a day is 4 kw a day and 4.2 KW a day respectively but how does the variation relate to plant metabolism?

I think you will see verrrry little difference between the 2 setups, if anything. A plant's ability for maximum metabolism is mostly based on the temperature in the room. In order for optimal CO2 to be utilized, you need about 85 degrees during the "day". The small light difference in question will not determine as much of metabolism as ensuring a proper environment 24/7. Once the right temp is reached, the max CO2 can be used, thus increasing the whole paradigm of increased metabolism and thus growth. Your lights are so close to base wattage, and overall KW delivery, in nearly the same amount of time that I couldn't fathom anything more than a 5% difference.