You will not receive the same brightness from the 400W that you would get from the 600W. Moving the light closer to the plant will take a chance burning the plant. Other things like node spacing and the ability to get light to the lower plant could cause you problems. If heat or power consumption is not the problem go with the 600W. I have been growing with 1000W for several years now after going thru the testing of different wattages and light sources, i.e. CFL, fluorescent, etc. The only problem I had to take care of was the heat generated by the light. You dissipate the heat and you never have to worry about light with a 1000W HPS/MH system. See sample just harvested. 5X5 tent.