Two entirely different cookers for entirely different things - The KJ is a higher end ceramic and is a top quality product, while the OKJ is a lower end stick burner made of thin metal, not that you can't get great results with it.
I've had 2 ceramics, and find they are versatile in that you can grill, smoke, and bake on them, but are lackluster in terms of performance as smokers, at least in comparison to an offset, which offers unparalleled wood smoke flavor if you're using properly seasoned wood. Also, an 18inch KJ won't give you much surface area, especially compared to the OKJ.
What are you planning on cooking on your new acquisition?