Neptune makes more stuff for the Apex, the programming on the Apex is more flexible, the Apex as a cloud interface that will run in any browser, I think the Apex is a bit more flexible. My apex runs the tank in the living room and some other stuff, ato water change rodi, in the basement. I am not sure the Reefkeeper can spread out over several rooms. When I was looking into a controller I thought I could use either one but the Apex offered more head room. The way my system is now I am not sure the Reefkeeper could do it.
That said. The Reefkeeper is much cheaper. Look at what you want the controller to do and what you might want it to do in the future. If the Reefkeeper can do it save the money. If the reefkeeper will not do it now or it will not do it in the future get the Apex now.