Yeah, that's how they "advertise" it and really, they can do that by offering a dual rate coil. The problem I have seen over the years is that once loaded up, the softer rate portion of the coils go solid and that leaves you riding on the heavy rate lower portion. Sure, you don't sag anymore but your ride quality suffers because of it. No need to take my word for it:
http://wayalife.com/showthread.php?12243-Rides-like-a-tank-RK-3-5&highlight=rides+tank
As far as adding control arms go, is there a reason why you're wanting to do this like setting your caster or maybe, are you planning on installing a new rear drive shaft? Or, are you wanting to add them because you think you need them for better flex? You do know that the amount of flex you have is dictated by the length of your shocks, right?
With the RK setup, you will still need to get quite a bit more stuff to bring up to the same level as the Rancho kit and so yeah, the later is a much better deal. I run one of their kits on my XJ now and am very happy with it. All their stuff is made in the USA and the quality is all there.