The 400 MH or HPS has the same footprint. That is if you use a reflector. By that I mean the space it light's up. In this case 5x5 area. That would be for both the bulbs,lamps.... You can use either lamp for veg. They really like the MH,but do well under the HPS. If they are small. I would wait till you get about 4 nodes,then keep the light about 18 to 24 inches from them.:thumbsup: