Depends upon what version you're talking about Karl, both basically do the same thing, it's simply a matter of the little things and support.
I have the RKE, and really don't care for it because it's not what I need now, I don't need pH control, I don't need oRP control, what I did want is the ability to record the history of what my tank is doing, but that's not a feature that was released initially, and who knows how long until it is, but it'll probably cost more money to be able to do it.