i think the metal halide is better.....the mercury vapor light can be used but only for a couple of hours a day in conjuction with an hps or another metal halide light.....on it's own a mercury vapor can't grow a plant, it can be used only as a supplemental light while in flowering, in order to make the plant produce more resin cause the mv light produces harmful radiation...
stick on the metal halide, it has blue spectrum which is essential for the plant