RK2 is the older model before Reek Keeper Elite came out. Right now if you're looking for a controller from Digital Aqautics your options are Reef Keeper Lite or Reef Keeper Elite. The RK2 is somewhat older technology but for something more than just your BASIC controls it's a solid controller.
If you never plan to need more than the RKL can do and only want a simple interface with simplistic screen then really no need to get anything more. But if you ever "Think" you may want some of the more advanced controls and a pretty neat interface/screen then you'll want to invest in the RKE.
Now if you want to move up to the "I can control everything in my tank, my house and my NEIGHBORHOOD" type of controller look into a GHL ProfiLux system.