How do you compare metal halides to metal halides? There are so many combinations of ballasts, bulbs, reflectors, you can't do it based on wattage, unless you are comparing similar setups, with varying wattage. You can get the same or more PAR out of a 175W as a 400W if the 175 is more optimized for PAR output. Same is true for any type of lighting, a photon is a photon, if you direct get more of them to your corals, and they are the right wavelength ("color") then you'll get more PAR. Really PAR and spectral plots are the two best tools to get a guesstimate.
Wattage is only useful if all other variables are held equal. It can give you an estimate of total output though, as most lighting technologies are similar in total efficiency, however, it will not tell you anything about how well that light is being directed towards your corals, or if it is the correct spectrum of light.
A better question would be, what type of tank do you have, and what are you trying to keep?