Dude,
A 600 will give you a lot better efficiency over a 400. If you look at lumens per watt, a typical 400 hps (I'm just going from memory here so don't hold me to this or bet any money on it) I think makes like 130 lumens/watt where your typical 1000w HPS makes 150lumens/watt. Kinda makes you scratch your head when people refer to grams per watt of yield. Different lights make different amounts of lumens per watt.
ANYWAY...a typical 600W hps produces the same per watt as a 1K. 150lumens/watt...IIRC
You COULD argue that a garden using 600s is more efficient than one using thousand watters because you can spread-out the light more consistently but unlike with 400watters, you still get the same lumen production out of a 600 as a 1000...PER WATT.
Heath likes the sixes (and gets outstanding yields) and DDs prefers thousands and gets outstanding results.
I'm on the fence. It's hard to argue with the growth that a 1K's intensity encourages. On the other hand, you CAN get better coverage with less smaller sources..and less VERY HOTspots (in temp and light).
Either way, 600s are much better than 400s..IMHO!