there is a point at which you reach no significant gain if your not using co2 the general rule of atleast 50 watts minimum per sq ft or slightly more is adequete for decent results, if your using co2 it would be very hard to actually have to much light.
the 1000 watt light offers some advantage over the 600 when you consider the lumens output of each the 1000 is by far superior and when you factor in light intensity at different distances the larger light will always be the better choice, but depending on growing style a 600 could do equally as well in a 3 by 3 space.
the efficiency of the 600 shouldnt even really be considered as its not significant and the lumens per watt of the 600 and 1000 is very close.