I wouldn't use any heater without a controller.
No matter what brand, you hear about most of them "breaking" and staying on, poaching your poor fish.
I bought a titanium heater with a controller built in at a local SW store for about $30. It is 200 watt unit, with a temp probe you place away from the heater to get an accurate temp of the tank. The controller does the work of regulating temp, so in theory the heater will last longer as well, and there's no chance of frying your fish because the controller just cuts power to the heater.
The brand I bought is Aqua Heat, but there are a few out there. You might want to look in to a heater guard for your sensitive discus.