Digital thermometer.

Normally, the dial in the heaters will be pretty innacurate, specially if it's set in a bad position in the tank.

The first thing you have to consider whan placing the heater is to have a good water circulation around it. I always set iyt as close as posible to the filter intake or retunr line. This way, I am sure water is contantly circulating besides. Doing it like this, also ensures a better heat dispersion trough the whole aquarium.

Avbout the themometer sensor, place it right in the middle of the tank, or in the filter current. If you put it close to the glass, you'll have a low reading. This is the main disadvantage of those stick-on-the-glass thermos.