Yes, the Via Aqua does hold the set temp when unplugged or during power failures. Texas has now gone the way of California and deregulated the power industry so that each year there are more power failures or as they call it "rolling blackouts". Funny how if nature does it, it is called a failure but if policy does it, it is called something else!
I never trust the setting on heaters as many are just more a ballpark setting than what actually happens. The reason for me wanting the sensor out of the tube is to get a much more straight reading on the temperature. With the controls inside the heater tube, the controls don't actually read the water temperature but react to the temperature the water makes the temperature inside. That might seem to be splitting hairs but not if you think about it for a bit. If you turn a heater inside a small tube on, the air gets really hot, really quickly. Hot enough to burn your hand. Since that is way more than I want my water temperature to be, the heater has to transfer that heat to the outside water or shut down. Right next to that heater that gets really hot is the electronic controls. One big enemy of electronic components is heat. If you heat little micro parts like diodes, resisters, and chips over and over each time the heater kicks in, they fail more often. I like the probe out in the water reading the temperature. With the controls outside the tank in a box on back, they are not getting reheated each time the heater works. A 2X4 inch box can hold components that are still cheap as they are not micro sized but can still be added to as the designer sees fit. Restricting the controls to a tiny board to fit inside the tube really crimps the design as far as what they can use.
As far as I have seen, the Hydor only comes in sizes which I have found to be dangerous if the contacts stick so the heater stays on.