The Cannabis Grow Bible by Greg Green:

All strains vary in THC levels and quanities or pecentages. The difference between THC levels and THC quanities is that THC levels are genetically determined: they cannot be influenced by the grower. THC quanities, on the other hand, are related to bud mass and how much resin can be collected from that bud mass. By way of example: some bud may only contain 20 percent THC, with a THC level of 5. The same plant grown under better conditions could produce 70 percent THC, with a THC level of 5. The level is genetic. The quanities 20 percent and 70 percent are under the grower's control.
I know for a fact that metal halides are designed mainly to encourage leaf and stem growth. High pressured sodiums are designed to promote flowering. With that being said, high pressured sodiums will encourage your plant to produce more resin (trichomes), and peak your plant's potency to it's maximum. But, if KanMan was correct, then my theory for his statement would be that the mh produces a higher THC quanity, but the hps helps the plant achieve it's maximum THC level. Fact is, that in reality, an hps improves yeildage, helps cannabis produce more resin (trichomes), and helps the plant to achieve it's maximum THC level, better than an mh.