I've done some research on them, as I'm planning on purchasing one. It seems that the general consensus is that the RKL is the more common 'entry level' controller, as it's cheaper than the Apex, a little more user friendly (opinionated), and the add-ons are a little cheaper. The Apex seems to be what most more experienced or high tech reefers go with. They can be programmed to do more things, and have more capacity for extra modules.
With that said, I really think I'm going to start with the RKL, but eventually explorer the Apex.
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