No, it's just that MH
has dominated the reef hobby in terms of lighting for 20-30 years (at least). PC
fixtures are relatively new, and in the past year I've seen companies double the number of models they produce, so they can cover freshwater planted tanks, and saltwater tanks.
It's all about spectrum AND intensity. MH
is nice because you get 250watts out of a single bulb. However I know that with FW
doesn't give off the most ideal spectrum. In a SW
tank I don't know if it matters as much, since you don't have plants with chlorophyll that need high peaks in red and blue spectrums.
Also, from what I've learned so far, 3-5wpg is the 'minimum' for a reef tank. at 4wpg you're supposed to be able to keep most soft corals. definitely over 5wpg for the stoney corals.
I don't think a 4x96 fixture will fit a 55gallon tank. I'm pretty sure that fixture is 6 feet long and has 2x96 on each side..basically made mroe for a a 120gallon tank.
I bring this up because on my 75gallon, which is 48" long, but 18" wide instead of 12" like your 55g...I'd be very hard pressed to get 3 bulbs over the tank, with reflectors, front to back.