Nah20 here uses the Tunze and loves it. I use the Litermeter3 and have had no problems with it. One thing I like about the LM3 is that it will only put in the amount of water its programmed for, no more.
This saved me once on a smaller tank when the skimmer lost a fitting and began leaking water into the stand. A typical ATO would have continued pumping in water to cover the loss, and the salinity would have nosedived, killing my tank. As it was, the sump level got low enough the return pump began sucking air and cavitating. The noise alerted me to the problem and I was able to resolve the issue w/o any problems to the tank inhabitants.
You can also set up a Tunze ATO to do the same thing by limiting the amount of water available to the ATO unit. This will however cause you to fill the ATO source water more frequently...but its a small price to pay IMO.
Nick