First off, I have Icons (coilovers and bypasses). As far as the major differences in the Kings vs. Icons (and Fox, for that matter) off the shelf, the construction and "quality" is the same pretty much, from everything I know, have seen, have heard, etc (I'm a suspension nerd). The difference is valving. King and Fox both use progressive valving, which is why they feel more "plush" most of the time and "stiffer" on big hits. Icons are "digressive" and feel stiffer on the road and small bumps, but feel "less stiff" when soaking up bigger hits at speed. As an FYI, either Kings or Icons can be custom valved however you want brand new or whenever you want to have them rebuilt.
For me, I like the digressive a setup of the Icons as I think it suits my driving style and how I want the truck to handle. If you can, drive both setups back to back and pick one. If you're in SoCal, you can probably do this for the cost of a round of beers for some friendly T4R members.