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