To make it really simple, if your planning on installing an aftermarket front bumper or an excess of added weight in the front end I would go icon as the dual rate coil will help hold the weight with less sag. (Won't be compressing the softer true progressive carli coil)
For a street truck with close to factory front weight you can run either and will be happy.
For someone looking for the "softest" or "spongy" ride I would go carli.
Putting a stiff shock with a carli spring will hide the softness and tuning in Carlis coils.
That said, a stiff shock will also make an icon run rougher than it could.
A soft or properly spec'd shock will always make either brands coil perform and feel MUCH better.
Both bands are plenty capable of making the average guy happy.
You can go wrong with either brand, and I don't want to make it seem like one is better than the other. Just depends on your application
These are all my opinions based off what I've ran, tested and reviewed. Along with customers reviews.
So take that for what it's worth.
Sorry OP if I got off topic, didn't mean to hijack the thread!