well the idea is, plants in the veg stage use much more of the blue spectrum of light. MH puts out primarily blue spectrum light so plants can more efficiently use the light. HPS lights put out primarily red spectrum which is used more efficiently by flowering plants. plants can thrive under both lgiht but it really comes down to how much of the light can plants use.

when people use 1 light the entire they choose hps because the plants spend more time in flowering so the most benefit would come from an hps light.