I've had an RKE and an Apex, and it is a complete no brainer, APEX ALL THE WAY.
The RKE is such a piece of garbage I am so thankful I took it off my tank. I have owned every single add on component and had nothing but failures. Seriously, I was literally one of the first 10 people to get the RKE, the serial number on my head unit was like 004 or something (that was before it broke, which DA says was lost in the mail, and I am out $100, but that is a whole different story).
DA has never done anything to help me rectify the issues I have had. Customer service and support is non-existent. Emails go unanswered for WEEKS. Your only chance is to talk to someone on the phone during business hours. I would not trust everything you are told, they told me I was on a waiting list for a replacement conductivity probe, I spoke to someone else and they told me there is no such list.
I've had probes not work properly, probes losing calibration too quickly, a probe that never worked (salinity/conductivity), which, btw has been broken since last December and they are still not shipping the replacements (check their website under the probe section). 11 months and still no replacement? Apex has the same probe (lab grade) and I have never had problems with it.
I've had a PC4 burn up (which DA is still thinking about whether or not they will replace), it literally melted the socket, the plastic was fused to my ballast power cord. I bought a replacement PC4 which was DOA (they have not shipped me a replacement because they do not have them!). (BTW a PC4 is a required component for any RK system and the fact that they do not have any should tell you something).
The components are very poorly made, I'm in engineering and I can tell you whoever did the design for the plastic housings clearly is not very good with CAD or they are made in a factory that has the WORST tolerances on the planet. Also their wiring system is a mess, cables everywhere and even if you buy the 1' cables it still looks like a mess (my Apex setup is so nice and clean).
You have to disconnect devices to upgrade them, complete PITA. You even have to disconnect the power strips from power before upgrading them or they might fail. This means you literally have to unplug them before upgrading them, not just the serial "bus" but the AC itself. If one part of the "bus" (I am using quotes because it is my belief that even with the BUS the system is very serial in nature) would fail, the entire system is down. Since everything is wired together, if one cable goes out and separates the headend from the PC4s, all the ports on the PC4s would be off (you cannot set default states, I hear you may now be able to do this in their v2 software).
The hardware in the system is really low grade, it takes about 20 seconds to retrieve 30k worth of graph data from the NET module (I set up a sniffer trace to watch why it was so slow). Seems to me like they went budget with it and there is only like a 32k data path at some point in the system. It is literally 1-2k per second until the graph comes down. Oh, the graphs use Flash so you can't view them on an iphone.
Did I mention that you can't program the system remotely?
Without going into all of the other problems I have had, I'll tell you that the Apex has been a breeze and is so nice and neat, I just love it.
Everything I have mentioned is absolutely true and I can back it all up. If you decide you must have the RKE, I have a ton of parts for sale if you need something (except PC4s and the salinity probe of course!)