I have read that the 315W could be the equivalent of a 500W HPS, have not heard 600.
From my experience I would say it is better than a 400 and not as good as a 600. Now that being said there or other factors to consider, a 600 is true watt, the 315 gives you say the equivalent of 500. So the power saving is substantial. With two 315's I get say 1000W's at 615 watts of power. Two 6's are 1200W of true power, and the wattage equivalent is damn close. Second to me is the heat, the 315 is much cooler.
Just some things to consider.
GR