The only time I would use 2 600s is when I have a long and skinny grow area and a 1000 watt just wont get the edges. However, having observed myself and others use 600s and 1000s...I will always use 1000s. The penetration like Known says is really spectacular. Those little bottom nugs make up a lot of the harvest and if I can double their size...it makes a big enough difference that the little bit of extra electrcity is well worth it. For SOG, I still think its better cause your nugs will just be that much fatter if yuo can get the light close enough.

I dont neccessarily think 1200 watts is better then 1000, for the plain fact of penetration, but Known has good points for why you would use them. 600 watt lights tend to run more efficient (not less total electricity used then a 400 watt, but comparitively its better).

1000 watts does also produce more heat. I think its harder to cool 2 600 watt lights then 1 1000, but for some people it might not be depending on what equipment they have. On average, Ive seen 1000 watt lights run about 10 degrees warmer then 600s (if they have a good fan). With a 1000, I need a 300cfm fan aircooling the hood to keep temps respectable if there isnt much airflow inside the actual grow room. For a 600, it only needs a 150cfm fan. I wouldnt do more then 8 smaller plants with 400s. However, without even an aircooled hood, you can get a 400 watt with 8" of the canopy no problem long as you have some air running between them.