In a word-uniformity.
Having more light sources of lower intensity rather than a single high intensity will give you a more even 'shower' of photons of light over your grow space.
I'd also use some common sense when comparing light fixture. Because you normally won't know the efficiency of the diodes used in the fixtures, ACTUAL POWER USED is the easiest way to compare fixtures. Unless you're using current tech electronics like QB's, strips or some cobs you will need the same amount of electrical wattage to get the same amount of light from your MH/HPS rig.