Watt for watt both MH and PCs produce about the same amount of light and heat overall,
I have to disagree with this. You can't just look at watts when you're looking at lighting...at least in reefkeeping you can't (the WPG rule doesn't mean jack when it comes to keeping corals). The differences between NO, PC and MH is largely a matter of PAR--basically, the
intensity of the light being put out. Halides generally outcompete PC's, unless you've got a 70w halide and 200w of PC. But, if you were to compare two fixtures of even wattages, but one had only PC's and the other was just a halide, the halide has an incredible amount of PAR compared to the flourescent fixture.
This is important in reefkeeping because many corals require intense amounts of light as they are found in brightly lit waters in the tropics. Because halides are a point-source light, they also create glitter lines in the tank as the light refracts through the bends and folds in the water's surface. This acts as a magnifying glass, and punches very intense amounts of light into the corals' tissue, etc. This isn't necessary for plants. We're talking apples and oranges here, even in regards to lighting needs.
Even though I'm a fairly strong proponent of halides over reef systems, I don't see much of a need for them over planted tanks...unless money isn't an issue or the price of a halide system is going to be comperable to a large PC setup. You generally have more heat dissipation to deal with (but that varies), if you're going to go with an HQI or DE bulb, then you have to make sure ot have some sort of protective glass covering between the tank, bulb, and tank viewers so it will block the UV being emmitted, etc.
You can do it, no problem (and the plants will LOVE it)...but it's not a requirement by any means.