What is the best model to get, for the money?
If your planning on controlling led lights or tunze pumps. Go with the full Apex.
The 500$ version comes with a ph probe also.
If your not going to use the variable ports then go with the Apex lite.
You can always buy the VDM module later if you want to control pumps or lights.
I love my apex.
At the moment I'm working on a water mixing/continuous water change system.
One day a week it will mix a new batch via delivery from a separate container full of a highly concentrated salt water slurry. This is done by using 120V solenoid valves.
I'm running a PM2 module to control the salinity and temperature also the valves in the new saltwater mixing container.
The rest of the week it will change out 1.5 gallons per day in my 100 gallon sps system.
My point is, the possibilities are endless with the Apex system controllers.
The best money I've spent for my hobby so far.
Happy Reefing