I believe 600w have the most lumen's per wat. Hmm it varies tbh dude and is a hotly debated subject among some.
A 1000w is going to cover a much larger area than a 600w, partly i guess since it can't be as close.
It all depends on the areas. I personally would rather use 2 x 600w in a space where a 1000w could go on its own.
A 1000w also produces more heat than a 600w.
You would see a large difference in bud production with either over a 400w.