Use both of them. Also, I can't believe people do'tdo this. When they are seedlings you don't need 650 watts to get tehm going. Turn off your HPS in this stage and go get yourself a 70 watt mogul based metal halide bulb and one 150 watt bulb. You grow in stages as the plant requires the light to go from a light to medium to intense discharge. It will save your energy costs over time. You can get these bulbs at home depot for 12 bucks.
