600s do put out more lumes per watt, about 7% more but just because they put out a bit more lumes per watt that doesn't mean they are better otherwise people wouldn't buy the 1000s. Yes 2 600s are better than 1000. 1 600 does not give you "the best bang for your buck" verses a 1000 though! because the 1000 puts off more lumes and with cooling you can hang them just a just as close! At 2 feet away the 1000w is just as strong as the 600 1 ft away! So using the inverse square law, it proves that the a single 1000 watt is still better bang for your buck than a single 600 even though it is 7% more efficient, you cant argue with science that has been proven as cold hard fact! It like saying the bmw 325 is better than the M3 because it is 7% (don't know the actual difference in the cars fuel economy just using it as a reference) more efficient. Yes it may cost you a bit more to run but you will be happier in the end. If I have the space for 2 600, I have the space for 2 1000s! they cost about the same to buy the stuff and the 1000s maybe cost 10 or 20 dollars a month more to run (at most 40 more between two ballasts) but will produce more than just an extra couple grams! not trying to hate on 600s! The only time I would run 2 600s instead of 2 1000s is if I didn't have a breaker that could handle the 1000s and all the other stuff, but it is cheap/easy enough to change/add a breaker. Just my opinion!i have a 400w, a 600w and a 1000w...right now i use my 400w cuz of space issues (3x3)..but if i had a little more space and lower temps i'd roll with my 600w and if i had more space i'd debate on buying another 600w instead of using my 1000w..600w actually give you "the best bang for your buck" what i mean by that is..They put out the most lumens per killowatt