Selecting temperature probe for home use with long cable / low power
Because you want to have 20m Cable, I would use a digital sensor. Temperature is measured most of the time with temperature dependent resistors so the cable would distort the measurement. 

Also you will have EMI problems with long cables!

P.S. You know that you can buy a cheap weather station which includes wireless sensors and a display unit? You will not get any cheaper if you try to build the system yourself. Maybe you can also find one with datalogging.