I would agree that supplying between the minimum of 50w per square foot of canopy and the maximum of 66w per square foot of canopy is best.
Now I know lumens aren't necessarily as important as PAR watts because lumens is a measurement of how our eyes perceive a liggts brightness/intensity and I believe we can all agree that the more intense the light (to an extent before reaching diminishing returns) the better.So with that said...
1kw bulb has 140000 lumens and a 600w bulb has 90000 lumens.To find the efficiency we look at lumens per watt.To find lumens per watt we divide the total lumen rating of the bulb by the wattage of the bulb,ie;
140000/1000 = 140 lumens per watt
90000/600 = 150 lumens per watt.
Hands down the 600w is more efficient.
Say for instance you use 2x 1kw lights over a given canopy,we'll say a 40 square foot canopy a.k.a. a 5x8 area.That gives you 50w per square foot of canopy and 7000 lumens per square foot,which is good,but you only have 2 sources of light/over lapping light,which is another good thing,but...
In the same 5x8 or 40 square foot area and utilizing 4x 600w HPS's you would be providing plants with 60w per square foot of canopy and 9000 lumens per square foot of canopy,as well as providing that canopy with 4 overlapping liggt sources,which is better.
Now can you see how a 600w HID is more efficient?