As always money rules. If all you can get is the 400 it's going to be good. More is always better but you have to be able to buy it. I have a 4 x 6 but only use 3 x 4 of it with a 1000 HPS. You can spread the 2 400's out a little, almost as good. If you go with insidesun and they have the option to get a hortilux bulb . Get it, it will be 40 bucks more but worth it.