Watts/gallon is kind of confusing. I think the rule was made up when you didn't have as many choices for lighting, as that rule has gone out of favor nowadays. The rule will get you in the ballpark, but it shouldn't be used as a hard and fast rule.
While all Watts are equal, the available light (pars) coming from those Watts for the coral is what is different. Intensity and the ability of the light to reach deep into the tank separate the different lighting types. T5HO is more intense than PC
and will reach deeper into the tank. In addition, T5HO bulbs are smaller and usually have better reflectors than PC
. All these things makes a direct comparison harder.
While I'm not super familiar with T5HO, I would guess that the two fixtures you're talking about would be comparable as far as the type of corals you could keep. It's just that the T5HO fixture would give you more opportunities to put higher light corals in more places in your tank.
I have the Coralife fixture you're talking about on a 46g. I can keep any LPS
, and some of the lower light SPS
corals just fine - as long as I place them correctly. If I was starting over again, I'd probably go with the Nova Extreme PRO (the one with 6x bulbs) or one of the TEK T5HO 6 bulb fixtures. You might want to look at the "PRO" version of the Nova Extreme. For a little extra money, you can really open up your ability to keep a bunch of different corals.