Measuring humidity isn't as simple as measuring temperature. Although you won't know it when looking at the typical data sheets and specifications for relative humidity data loggers and sensors.
This is one parameter where you can say "Not all relative humidity sensors are created equal". Companies like Testo manufacture their own relative humidity sensors in house, while others like Vaisala and Graphtec work closely with suppliers.
The issue we have seen with the cheaper relative humidity sensors is accuracy and drift. Many claim they are ±2-3% or ±5% accurate, may be they were when manufactured, but in the real world, they soon begin to drift and the accuracy blows out.
If you just need an approximate idea on what the relative humidity is, then any sensor will do. If you like something a little better, then stick with some of the brand names who have put in the effort and the engineering hours.