Ohhhh ok gotcha
Now hmmmmmmmm

I did lots of reading before I made my mind up. There are differences for sure. What made my mind up in the end was the layout of the PC4 bar vs the Energy8 bar, the light controllers, and a couple other things. Don't remember right now.
I'm just trying to help you become more informed.
You won't hate the RKL, or the RKE.
Many would bash me for not selling you on an Apex.
It's a great unit too. I didn't like the fact that the starter unit was over $300.
There are others too, such as Profilux and another that isn't coming to mind.
$130 is a great starter price, but inform yourself first.
And keep in mind that I didn't cheap out, I just bought what I liked. It still cost me over $500 for my RKE with some other goodies, and I started with the same thought you have.

http://www.ghl-direct.com/

http://www.neptunesys.com/

http://www.digitalaquatics.com/

if i were to start with the RKL and down the road want to up grade to the RKE would all the addons still work fine

Mostly, except for the iTemp, which comes with all RKLs, and what I mention near the end.
The RKE comes with a different probe, just called Temp, that has a different connector.
The RKE can also use the newer SL2 while the RKL can only use the SL1.
The SL1 is compatible with both and has connections for 2 switches, a Temp probe and an ORP probe, plus 2 com connectors.
The SL2 has removed the OPR port and replaced it with a Salinity port.

You should double-check on the DA site though as I am not 100% sure about the Advanced Light controller, and I believe the Net module only works on the RKE.
Sorry if I missed anything. I'm still new to these controllers.