You should be fine with a 400 watt but if you can swing a 600 then I say go for it.
The price difference is not that dramatic and you will get some added benefits like greater light penetration. I think with a 400 you can get 21" depth and a 600 will give you 29".
I usually prune my plants to try and keep them around 24" and my 400 watt does the trick although I kinda wish I had a 600