Adding a vote for the Tunze nano. Yes it's over 100 bucks. But it doesn't have an annoying sensor.
Here's the thing: You're adding an ATO to take away the brain real estate of having to top off the water every day. Does that add to your maintenance and create another fail point? Sure - but ATOs are luckily very mechanically simple or use fairly few components, and they're a bit noisy. So when they're not working, you can tell, and you can usually fix it easily, or know what's specifically wrong. You'll notice your water line hasn't gone down, or you haven't heard it in a while, or your salinity is off. IMHO the pros vastly outweigh the cons. In a 40g I'd be more worried about my salinity fluctuating with evaporation.
Furthermore, when your ato is malfunctioning, it's not a tankwide disaster, say like a calc reactor or overdosing 2 part. you will have either a dip or a spike in salinity depending on what the issue is. But as long as you take your time when installing and read the directions/warnings, you should have no trouble.
Obviously if you dont notice it not working for a week, that is a larger issue, but I would say it goes off enough throughout the day that you would notice if it wasn't working that long.
Just follow the directions, perform periodic maintenance on it, and check on it once in a while and you will be set.
