Which one should I get?
DHT22 has higher precision and support negative values.
Both work with my library - Arduino/libraries/DHTstable at master · RobTillaart/Arduino · GitHub
But is it worth paying 3 times more?
But is it worth paying 3 times more?
Depends on your requirements and budget. Personally I have one or two DHT11 to maintain/test my library but I prefer DHT22 over DHT11 in projects because of that extra digit.
A very good sensor comparison is found here - sensors:temp-hum_sensors_compared [Boxtec Playground]
It is in German but you should be able to compare the graphs and draw your conlusions.
The DHT22 is 2 dollars on Ebay : dht22 for sale | eBay
The DHT22 is very inaccurate for the humidity, it can easily be 20% wrong (or more). For serious humidity measurements a better sensor is needed. They cost more. Here is a list of common humidity sensors : Sensors, Humidity Products Category on Adafruit Industries
The DHT11 is 1 dollar on Ebay, but it can only be used to learn about Arduino and sensors. The temperature is not very accurate and the humidity can be 40% wrong (or more).
I use sht21. They cost around £2~5 on eBay and use i2c interface, so you can connect them to the same pins as bmp180, your rtc etc, etc...
Accuracy... well, alot better than dht11. But not perfect. I have two next to each other on my desk right now. They read around 0.25C different, and around 2% difference in humidity. I will buy a 3rd one and see if it agrees closely to either of the other two.
Paul