The heater is the single most important piece of equipment for an aquarium. Don't skimp on heaters. All it takes is one failure--whether the heater fails to heat during the winter, or it overheats--and you can have a tank of dead fish. Filters and lights failing will cause no immediate harm and can be repaired. Not so the heater in certain circumstances.
I would not go less than 200w and I would have two of them, one at each end next to the filter intake at one end and the filter outflow at the other. Alternatively, you can buy a canister filter that includes a heating element, avoiding any in-tank heaters.
Cost is not always a guarantee of better, so know what you are buying.
And the comment on ambient room temperature affecting heaters is very correct. In a cold room heaters will have to work longer to keep the tank warm. Larger tanks retain heat better due to more water volume, but they also take longer to heat up should anything go wrong.