In my system, the controller sensor and the heater are located to facilitate water changes.
The sensor probe is in the main aquarium to monitor the temperature drop when I'm performing a water change. It usually drops 1 degree (77 to 76) while I'm mixing & heating the salt solution.
The heater is in the return compartment because that is the largest compartment available for mixing and heating a salt solution. When the LED in the calibrated heater turns off (79 degrees), then the salt solution is ready and I turn on the return pump.
While the salt solution is being mixed, I use a powerhead to stir the salt and ensure even heating.
The overflow is a bad place for either the heater or probe because it is emptied as one of the first steps of the water change. By pulling the standpipe, it empties into the sump which is completely siphoned into a basement sink.