Apex Lite is awesome. There are only couple differences between the lite and the apex full version.
One is the additional ph/orp port. The variable speed control if you plan to use tunze pumps etc and dimming feature for vho lights etc.
The variable speed control would not be a factor if you use vortech type of pumps and most of the newer leds on the horizon have dimming in them like the AI leds.
the extra ph port well I did not need it at the time so I went with the lite.
other than that there are no limitations with what the lite can do. they both have access to the same software with net access and same modules for expansion.
the RKL/RKE's are also nice. They have some features that the apex cannot do and vice versa. the Net module should be a lot more useful when they finish and release version 2 of their software management.
Whichever system you go with you will be happy with. they both are good.
if you are on a budget the RKL L1 can't be beat at $99.00. it gives you 4 power ports to control (PC4) and it gives you a temperature probe (iTemp) to monitor and control your heater.
But if you needed more from it then you would have to end up getting the additional modules like the SL1/SL2( for ph, ATO, orb, salinity etc) and another pc4 to control more outlets. if you want NET access thats another module you would need. by then you are in the same price point as the apex lite, which I think is tad superior to RKL due to a better display at the control.