just my view, but I think two lamps are better than one, for more even light dispersion, and lumens may be higher with two lamps ... go to http://www.1000bulbs.com/ and search their specifications as to lumen output (good place to buy, also ... check out their odor-neutralizing CFLs too, they WORK !!!) ... one thing to consider is price ... in my price searches, the 600w lamps are priced much higher than the 400w, and not that much cheaper than the 1000w lamps ... if you can afford the electricity, I'd take the 1000w over the 600w ... where I live, my 400-watt sodium, fans, etc., cost me about $22/month for electricity, no big deal ... :smokin: