600 would be fine, probably easier to vent the heat out, but 2-400's would definitely be better simply because there is more light. Always remember the more light the better. Either way on a space so small I would suggest a Cool Tube or a hood that has a vent built in.
How tall is your space?
EDIT: well I would listen to Akorn, his post wasn't up when I put mine in.
This morning I ordered 2 400 watt HPS fixtures with ARGO conversion MH lamps.
I know there is always a differenec of opinion on ANYTHING and I appreciate all of your advice. Some points well made and taken.
I have 8 feet of vertical space with an attic directly over head. 2 feet away from the closet is a window with a huge Air Conditioner in it. I use that for cooling the whole room. I have a couple vents cut into the wall with small fans sucking cool air into the closet from the main room. I'm gonna vent the room or haet and odor through the attic and tap into a heater pipe that penatrates the roof 2.5 stories up. Also, the closet sitts on the eastern side of the Townhouse so the sun is hottest on the other side of the home late in the afternoon. I'm gonna run the fixtures from 7PM to 7AM using the cooler night air to my benefit.
I finally decided to go with the 400s so I can really get those f@$#%s tight on the tops. Each fixture will cover 4 square feet for total of 100 watts per square foot not including the overlap from the adjacent fixture. I'll have 4 plants per fixture. Eventually hoping to harvest 4 plants every 30 days.
Thats the plan anyway. We'll see. Plans always change...........right?
the actual closet is 29 inches wide by 8 feet long. I split it to make 2 rooms. One veg and one flower. The flower will probably end up being a bit larger. Like 5 feet flower and 3 feet veg.
bah, I woulda got 2 1000 watters just kiddin. The one thing I plan on changing in my room is the lights. I started with all 400 watters and 1 430 watter. Next grow I'm gonna get a 600 watt electronic ballast and see if there's a difference (I'm sure there will be). 400 watts just doesnt seem to be enough. One thing I would suggest to you is, get yourself a decent fan and vent both of those lights. That way you can keep the lights right on top of those plants. I can keep my lights within a few inches of my plants with no problems. Good luck, I look forward to your next grow with your new lights!