Unfortunately, there's almost no way to generalize this and it comes down to questions of livestock needs (and future plans) and aesthetic preferences.
100w
MH doesn't equal another 100w
MH either, unless the
ballasts and the bulbs match.
PAR (Photosynthetically Active Radiation) and PUR (Photosynthetically Usable Radiation) can vary even within a bulb spec (i.e. a Hamilton 250w 14K bulb and an XM 250w 14K bulb probably don't have the same ratings). Even with the same bulbs,
ballasts can figure into the equation as well. I'm not nearly as familiar with variances in
PC lighting as I am with
MH lighting, but I'm almost certain similar effects occur.
Take into account the physical nature of the light.
MH = point source of intense radiation.
PC = elongated source of diffuse radiation.
The only thing I can consistently say is that
PC will evenly light your aquarium (assuming your bulb arrangement somewhat closely matches your tank length), but
MH will more intensely light your aquarium with fall-off towards the edges. That's speaking X watts
PC vs. X watts
MH.
The harmful UV light put off by lighting should be blocked by protective glass over your fixture since it's not good for the animals either.
The short answer to your main question is "No", but that wouldn't have been a very polite answer.
