A 1000W HID would be perfect for that size room, as long as you can keep things well ventilated and cool. But if you're leaning between the 400 and the 600, I'd go with the 600 too. That's like an extra 15,000 lumens or so, and the plants will take as much light as they can get. Remember to spend about as much on ventilation as you would on your light, which will alleviate a lot of problems right off the bat. Good luck!