id jus go with the 600 watt its nearly twice the lumens for only 200 more watts they have the highest output per wattage when compared to 400watt or 1000watt hps
if your grow area is small the get yaself a good extraction fan i had a rvk100 the smaller cheaper version (175m3) in a realtively small area and it worked fine, i did have the same for intake to though.

the 400 watt does produce less heat and it is noticeable differance so if you cant use an extraction fan the stick with the 400 as you may get some slight heat probs
oliwog Reviewed by oliwog on . 400w HPS vs. 600w If I'm gonna have a grow room approx. 4W x 4L x 6H, and perhaps 2-4 plants, would a 400w or 600w HPS be better? I don't wanna buy a 400w and then always be wondering if I should have gotten the 600w... Rating: 5