k, also, I should have mentioned this is a closed setup with an A/C unit. so, I'm not concerned with heat using 2 400w or the 600w. now, I'm aware the 600w HPS is a bit more efficient than the 400w, I don't know how much more efficient tho, and I do know the 400w was the most efficient previous to the 600w invention. Also, I am a believer in the light rail as I've seen what it's done for a friend's setup - it increased S2 growth and yield by about 20%.
so, with this new info on the room design using A/C, do you still like the 600w with light rail over a pair of stationary 400w's ????