aero king is on point,, i have used 600s and 1k's, but its all about ur setup. if your gonna air cool, and how much money you have to start with. 1ks come out cheaper in the long run, it takes a LONG time to come back into the cost of an extra light in 3 600w setup compared to 2 1ks. an before your even close to making your money back in the 200watt an hour saving youll have to buy new bulbs. so more money again... like aero said, big or small plants...
600w are cooler temp wise, but i find 1ks give tight hard nugs, even on tall 3 footers,,