Its about wattage and how the bulb pushes out the Lumens. 54 watts can only push the light so far, Just because you have 10 of them dosent mean the light will travel as far and be as powerful as a 400watt HID light, were 400 watts can really pump out the lumens. The light is a lot more concetrated from the 400watt HPS.
i just posted this on a different post.. but this is the break down of lighting. if you can prove me wrong ill go HPS. only thing i cant find is what spectrum does for a plant. if it makes the yield bigger or what.
Theres a few things when it comes to lighting and i know for a fact alot of people never looked this deep into it. theres lumens, spectrum, PAR.
now based on a 50,000 lumen out put 1. 400 Watt HPS = 10. 54 Watt HO fluors.(1st pic)
now theres spectrum which this is where i can not find any type of information on how the plant uses a 2700K spectrum vs a 2100 spectrum. basicly how does the spectrum effect the plant.hps about 2100 and fluors about 2700K.(2nd pic)
then theres a PAR rating which is how strong the radiation comes off the bulb.. if to close it will burn.. kinda like when you goto the beach and in the summer you can feel the sun burning you cause its closer then it was durin the winter time (par-Photosynthetically Active Radiation). its just the radiation that the sun/bulb puts off..(3rd pic)
Now that you kinda have what im saying down.
I know a HPS can power light down to the lower branches really well and needs to be 3ft-4ft away other wise it will burn(high PAR rating). And since the fluors dont have a high PAR rating the rays it pushes out are weaker. so you can get the bulb inches way from the plants before they will burn allowing rays to penetrate to the lower leaves just like an HPS.
Hands down watt for watt the HPS is more efficient.
1. 400 watts HPS 50,000 lumens.
and
10. HO 54 watt fluors=540 watts using 123 extra watts to make 50,000 lumens.