Not sure how the Hydor sensor works, just seems to have good reviews. It doesn't come with a pump though. Saw on Amazon you can get it and package it with an airlifter pump and pump mount for 100 bucks. I think I go through about a half gallon a day for evaporation. Really annoying when I'm traveling and have to ask my wife to put some water in.
The hydor utilizes 3 temperature variation sensors to determine your water level and activate the ato pump. Only issue I could see with this is if your ambient air temperature actually equals your tank water temperature which could occur in some people's houses (I'd say most not) or in the event of broken ac etc.
I strongly recommend an auto top off for the reason you described as well as the added benefit of keeping your salinity as consistent as possible.
Most ATO use float switches and any that do not (or only use them as a redundant fail safe) are far less prone to failure in the on or off position and far superior for that reason in my opinion. This is especially important if you utilize kalkwasser or lime water in your auto top off as a failure in the on position will result in an overdose which can be devastating.
I personally recommend the Tunze universal Osmolator 3155 as it has never failed in the on position in my experience or anyone else that I have heard about. I have had float switches and even multiple float switches fail simultaneously. The less moving parts the better. Osmolator 3155 uses an optical eye, back up float switch, and back up set timer to cut it off automatically if both above fail.
If the Osmolator is currently not in your budget, the hydor seems like a good alternative and I definitely recommend it over a standard float switch auto top off. Having said that, I have no experience with it and don't know much else about it. In any scenario, I recommend utilizing a redundant fail safe (or several) whenever possible.
There is another temperature based ATO out there that I know is good but I forget the name at the moment.
