I sure wish I hadn't decided to delete Aaron's quote to me explaining the difference between the two when I asked the same question trying to decide what to get for my Jimmy.
Basically he said that the ICON can deliver lots of solid low end response, resulting in lots of smiles and that the XCON is similar but with a stronger more direct impact. He went on to describe them in terms of heavy artilery. The ICON being the shockwave you feel from the explosion and the XCON is the missile straight to the head.
Damnit I sure wish I still had that quote, the way he said it was awesome.