This post may be a little late and a dollar short, but i thought i would give my 2 cents in regards to your dilemma of running a 600w or 1000w setup. I myself have had the same dilemma and after a lot of research, I have come to the conclusion that obviously a 1000w setup will have an overall higher lumen output, however, you have to take into consideration how far away that bulb is sitting from the canopy. In my own setup, I was either going to run 4 600w's or 3 1000w's, but after a little testing the 1000w's are much more difficult to cool, meaning they have to be placed much further away from the plant causing a larger amount of lumen loss. I ended up going with 4 600w, and I am able to keep the lights safely within 4-6inches without any problems, although I choose to keep them about 8inches just to be safe.
My point being, when taking into consideration that you rarely ever have optimal conditions to keep the light cooled and as close as possible to the canopy as possible, you are never gong to be able to fully utilize a 1000w. Also, taking into consideration of height issues, like myself where I am limited to about 5ft from ground to ceiling, I would be very limited in canopy height with 1000w, being that the light should be safely 18+inches away from the canopy. Take the fact that I can safely place a 600w within 4-8 inches of the plant, my lumens are much higher because of the distance of the bulb to the plant and I only lose at most 8inches of possible canopy growth..
At one time I had all the calculations for my own study and could show you my results, but I have since lost those, but there are plenty of charts out there that give you the lumen counts based on the distance from the canopy for each type of bulb.
Like I said, just my 2 cents and I am probably a little late, but I would recommend to anyone considering running a bigger bulb to always take into consideration the lumen loss based on the distance from the canopy. My results aren't exactly scientific, but logical in my mind. I have run it by many grow shop owners, as well as fellow growers and it seems to make sense to them.
Also, I have had a couple people state an argument that 1000w's, even though the distance between the light and canopy are greater, the 1000w will have an overall better light penetration over a 600w. Thus, giving you more light to the lower canopy and better production in the lower canopy. Now, I have not seen the proof in the pudding on this, but definitely an argument that I am willing to listen to, if someone either had the results or some numbers.
Anyways, I am just rambling now, and probably typed too much and hopefully it makes sense at this point, but thought I would throw my 2 cents in and hopefully not hijack your thread.