Are those different rates/colors the same free-arch or different? The result would then determine how much remaining arch is left after the weight of thw car is upon them... and the flatter the compressed arch.. the better for handling. Again, knowing the rear weight of the car should be a determining factor in choosing the springs.. and how much remaining compressed arch are you willing to drive with (pending your driving purposes).

Actually, if you aren't going to use the car for serious handling desires/intentions, then you really don't need to step over to the fiberglass mono leafs. Their biggest advantage is weight reduction.