probably both the density and yield would be better with the larger light. im not sure about cost, since you would have to figure in the costs of keeping the temps down, more fans ect. i wouldnt think 400w would be to expensive to run, but i havnt got mine yet. i only have 150w for now. the bill isnt noticibly different, 5 to 10 dollars a month more than without. so guessing 20 to 30 a month depending on if youre vegging or flowering.