Another important piece of the equation is how linear the rates are. My experience with the stock springs is that the spring rates rise dramatically as the springs are compressed. On a rough road, this can lead to a feel of "bottoming out" on bumps and a very bouncy ride.
I have springs with a 700 in the front and 900 in the rear, but because the spring rates stay pretty constant throughout compression the car bounces less over bumps than with my stock springs.