Temperature sensors come in many shapes and sizes. They have very varied pricing and also features. I have tested several to see what the differences between them were. The DS18B20 is a one wire sensor meaning that you need normally two connections for the power supply and one connection for the data. The SHT11 works with an interface which is a lot like but not the same as a I2C interface. The sensor can measure both temperature and humidity wheras the DS18B20 is just for temperature measurement.
The SHT has an accuracy of +/- 0.3°C at 25°C and the DS18B20 has an accuracy of ±0.5°C accuracy from –10°C to +85°C. So actually as temperature sensors there is not a great deal between them. As a third sensor i would like to get my hands on the TMP35 which also seems to be one of the most used ones. The SHT11 uses 4 pins including the power supply and there is a parasitic power version of the DS18B20 chip which only needs two wires because the power can be gotten from the data wire. There is also a S version of this chip called the DS18S20 which is only slightly different from the DS18B20 version. Please look at the link to find the differences. For the DS18B20 i have made a small PCB which you can use directly plugged in to the arduino. The power can be supplied using two arduino pins so that there is no need to have a separate power supply for the chip. There is even a jumper onboard to switch between the parasitic power version and the regular chip. How easy can that be. If you want to read more including the software and hardware connections then please click one of the links below.