One will need more watts to achieve same amount of light. And if more power is used more heat will have to be dealt with. I, for instance, do not care much about how much power I use on growing, so If I'd be in OP's place, I'd put up as much MH as I am able to cool down. (or as much as plant is able to use).
Although MH are better than regular incandescent, they emit more heat than light still.