To be completely honest, neither are very reliable.
The big question on the dial type is if it has a cardboard baking or metal. The cardboard backed dials start to read further and further off as the cardboard swells due to the moisture. Metal backed dials are much better, and usually have the ability to be calibrated (using the salt test to calibrate it)
Electronic sensors have a similar issue. The older they are the less reliable they get. Even still 5% off each other is actually very good. Both devices probably have +/- 5% or even +/- 10% error built into them.
Accurate thermometers and hygrometers are very expensive, hundreds to thousands of dollars. This means that most people in the reptile hobby aren't going to be able to afford them. What people need to find are precise (see precision vs accuracy below) thermometers and hygrometers that can be relied upon to give consistent readings. Even if they are incorrect. That way you can over time learn how far off the readings are and compensate for them.
Precision vs Accuracy:
Accuracy is the measure of correctness, in terms of the picture closer to a bullseye
Precision is the measure of consistency, in terms of the picture closer groupings.
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