As far as harvest weight vs. electrical costs, my bet is shorter veg time, more plants. Also the less veg time the quicker the return on investment.

Here is my best guess based on my experience. I seem to have average results.
To fill a space 2 x 4 so it will gross a pound of bud with six plants you may have to veg for 12 weeks. Veg is 18 hrs/day of light = expensive. Same space 15 plants to produce a pound, 6 weeks of veg. Saying the same amount of soil/nutrients 15 plants x 3 gallons or 6 plants in 7.5 gallon pots. The bloom time is the same either way. You do the math.

Your results may vary. Some of the best growers will realize 2+ lbs for 6 plants, 600 watts in a 2' x 4'. Some of the worst will realize ugats.