600s are the most efficient lights on the market period. than they offer the best lumen per watt ratio of any other light out there other than possibly the 750s which i have not looked at.
also you have to understand that there is only a certain point where no matter how much light you give the babies they will not get larger without c02 because they are using all of the possible energy they recieve in the given amount of space.
that being said with your space you will not be able to use the full capabilities of a 1000 watt bulb. ideally you want to be between 50 and 75 watts per sq foot anything more than that and it is pretty much wasteful unless you have c02 to speed up photosynthesis.
i have personally run a test where i had 2 4x4 tables, one of which had 2000w focused on top of it , and another one i had 1000 watts over it. both tables ran exactly the same strain, 36 plants in each table, flowered from 10 inches tall and lolipoped
the table with 2k only yielded about 2 more ounces than the table with 1k because most of the light was being reflected off of the plants because they were using all of the energy they could in the space provided..
the moral of the story is yea you can get the 1k, but unless you plan on expanding in the future it would just be a waste of money.. if 10sq feet is all you will have to work with for a while then get the 600.. it will be easier to cool, you can get it closer to the plants, and you will save in electricity. the only way i would get the 1k is if you had money in your budget for c02 and were going to seal the room or tent and do it right