Go with the 600, yes you'll get a better yield. A 400 is better suited in the Homebox L(3.3'x3.3'), which is what I have. I'm running a 600W light if that tells you anything.
My first go-round didn't yield much, about 5oz. I pretty much lost half my crop due to numerous noob mistakes. My current grow is almost finished. I'm hoping to do 8oz. I plan to double that for my next grow. Eventually I'd like to clear 2 pounds. Until then, I'm just happy to have plenty of nearly free smoke.