It's all about who YOU are. Confused let me explain.
Neptune Systems Apex controller is a great system they have been in the controller business for years. Their systems are very reliable and tested. If you are into writing code and programming computers Apex systems are for you. Apex has plethora of options, probes and modules. My favorite being the Wireless Expansion Module (WXM) which controls your Ecotech power-heads. This alone would of convinced me to purchase it, however I did not know about it when I purchased my current controller.
Digital Aquatics RKE/RKL is also a great system in the way they have come about making controllers for the average person. To program a RKE a simple drop menu is used, ie IF your temp is XX then turn channel XX on. Although they do have many manufacture recall due to some bad planning and unreliable equipment. They are also not known for support. Usually if you have a question you are referred to their forum page, not always but most of the time. All of which is being corrected and has improved.
I have a RKE and RKL. It has made my husbandry for my reef tanks a whole lot easier. My RKE has automated almost every thing on my 72g bowfront. With the exception of my ATO. My RKL is controlling my BC29 Mod.
All in all they are both great systems, able to control every aspect of your aquarium. Both system allow you to control, receive and change system setting online. On your home WAN or outside it. A big plus if you want a quick update of you parameters.
Well I hoped I helped.