Rena makes a good heater if you can find one. Jager is good as are the ViaAqua titaniums. I heard of some problems with the Cobalts early on but they appear to have corrected those issues with the newer units. Cobalt is a good company and would recommend their equipment.
Completely agree with using a controller with any heater if you can as a fail-safe.
Size wise, don't fall for the wattage requirements on the back. If you keep your room temp around 72ish degrees, you are only looking at a 6 to 8 degree difference from where you want your tank to be at. Unless you live in a cold climate and don't run your heat, there should be no need for anything greater than a 150W heater.