Definitely buy them both. I have had awesome luck with a 400, but the 600 is better although they get hotter. You wont want to put even a 400 in a 2x4 unless you have fresh air coming in and the warm air exhausting at a pretty good rate. If I had the chance I'd buy both. If you have to pick one the 400 is more versatile imo, but you will more than likely want to supplement it with other lights if it will be in an area greater than 3x3. But the 600 is a lot of light for the money.