Yeah I went with 400 watts cause I got a great deal on a system, and it's workin out great so far. I guess 600 watts is a little more efficient but either 400 or 600 has a good lumen/watt ratio. I'm only growin one or two plants so for me I went with the smallest light that had a good efficiency and efficacy. I've seen some very good harvest under 400 watt lights.