Personally I'd like to have both lights the same wattage so I could use a flip flop switch & a single 600 ballast for both lights,but since you don't I'd use the 600 in the last weeks of flower & I'll tell you why.
The more powerful light will penetrate deeper into the canopy & since the plant is at it's maxium stretch hight wise,it will fatten up the lower bud sites better than the 400.
That's how I see it,hope that helps & good luck & good toking.