Metal Halide bulbs put out a bluer light spectrum that is most needed for plants during their vegetative growth stage. Using one will give you tighter nodal spacing (less stretch than will be seen using an HPS bulb alone) and thick lush growth.
 
High Pressure Sodium bulbs put out more light in the red spectrum range, that that is most needed by plants while in the flowering growth stage, but if used for vegetative growth will cause wider nodal spacing and growth that is not as thick and as lush as a MH bulb would give.
 
What can be best is a combination of both light spectrums. Some wattage HID lights can use a dual-arc bulb that puts off both a MH and a HPS light spectrum from one bulb at the same time. An example is a 100-watt dual-arc bulb is a 500-watt MH and a 500-watt HPS in one bulb.
 
They are not available in every wattage though. I have seen them in 1000-watt, 600-watt and 400-watt but lately for some reason I cannot find the 400-watt bulb offered so I do not know if there was some design problem and it is no longer offered or if the sites I had found it on in the past just no longer offer it possibly due to lack of sales.
 
If you can use one, it would be a wise option to take. If someone is burning lower wattage HID lights and can deal with a slight increase in temperature adding a second light of the different light spectrum is a good option to take.
 
If someone uses a different style of reflective hood depending on its size and design a second bulb of a different light spectrum can be added. Some reflective hoods are designed to accept a second bulb though most are not.
 
If someone uses CFL’s it is just a matter of purchasing ones with different Kelvin temperature/color ratings and then spreading them out so their plants receive as equal of an amount of each light spectrum as possible.