metal halide question

just started my first grow room and new seedlings r under florescents right now about to switch over to MH, my question is which is better for the plant 400w MH or 1000W MH. I dont want to over kill or under kill. from experience which should igo with. I will have 20 plants and will have 2 lights going. so 400w or 1000w? thanx bros

20 plants how big are you going to let them get? How big is your grow area? On they are movers or stationary? Are the hoods air cooled? Do you have temp problems? Adquate venting?

im going to veg for 2 months. venting is not a prob. my grow room is in a 12 by 12 room. the hoods r not aircooled but will have an AC set up to cool the room.