i preferred aquahub over autotopoff simply because it was easier for me to see what i wanted there, plus they have online guides that explain exactly how to set them up
i like this one:
http://www.aquahub.com/store/product27.html
either way, i suggest that you have a reservoir that holds several gallons of water that you either ensure has water in it or have it controlled by an ato with a latching relay so it will be close to being empty before being refilled. this helps with the TDS creep that can happen if your RO/DI is running for short periods of time and frequently. because of this your DI resin will last quite a while longer.
in my setup we have RO drinking water for the house with the holding tank and it splits off to feed the DI cartridges. it then goes to a solenoid which is part of the ATO system which keeps the reservoir filled between 3 and 18 gallons. there is a low and high float switch which latch to each other so if they are both down the water level is low enough to be refilled and will not stop until both float switches are floating. this is fed through a float valve for redundancy and a failsafe.
the solenoid is a 12vDC so low voltage there.
the tank(sump) ato is tied to this as well, with another relay and float switch in the sump. again, low voltage is used and i use a Tunze osmolator pump (12vDC) and again it is fed through a float valve for a failsafe.
all of the 12vDC is fed from 1 power supply so it keeps everything simple.
if you are just going to buy an ATO, I have never heard anything but praise for the Tunze Osmolator.
good luck with whatever you decide upon.
