Ok, my mistake, I should not have been so gun ho and said, 600 all the way, it really depends on your set up to be honest.
However, Snow Crash I still think your argument is flawed, cost comparisons should be done over longer periods, not on initial outlay. Don't use vague words in an argument like Almost...nothing like a politician to fudge some figures by leaving some area's grey yet covering them up by quoting %'s and kw to bamboozle (shampoo commericals have never fooled me either)....you then come out and say they "grow larger and denser buds"...based on what evidence?
My 600's grow amazingly dense buds and I have the pleasure of being able to compare them (like you guys in the US), to thousands of different types of bud.
I can also say everyone I come into contact with recommends 600's as well...where does that get us?
To the OP, look at what type of set up you have, assess what money you have, think about the long term (are you doing 1 grow, 2 grows, growing forever..) Do you have a small space with a lot of height, do you have a big space with no so much height, what is it you need to achieve (apart from growing some plants.)
Peace out DST
Because a 1000w ballast is ~$130 while two 600w ballasts are ~$300... Add in the cost of an extra reflector, and an extra bulb... Oh, and did I mention that 1000w bulbs (despite being less efficient) grow larger and denser buds? Two 600w bulbs cover a what, two 16 sqft areas almost as well as one 1000w covers a 25 sqft area (5x5)... So you get 7 sq ft for an extra 1.4Kwh per day plus the cost of the equipment, unfortunately the grower is using 38% less energy and as a result the buds suffer.
@~24" a 90,000 lumen bulb (brand new) would provide at most 7,3000 lumens to the edge of the canopy (1/12.457th).
@30" a 145,000 lumen bulb (brand new) would provide at most 7,500 lumens to the edge of the canopy (1/19.464ish).
So both systems offer enough energy for photosynthesis but the 1000w system provides heavier buds. Everyone I've ever come in contact with who has used both 1000w and 600w systems will suggest the 1000w. I don't know many that go backwards and think they get better product.