EHEIM Jager heaters are by far my heater of choice. They are a bit expensive, but it is well worth it. I have the 100 watt heaters in each of my 10 gallons and my 20. The temp never fluctuates, and they are able to be re calibrated! So if the temp is off when you first get it, you tell it what temp it really is heating to and then readjust the temp to the one you want. No need to set it to 75 to get it to heat to 80. I had that issue with another heater in the past.
Also, they last forever! I had a 150watt in my 40 gal for over 4 years. Then I tore that tank down, stored the heater for 3 years, and it still works... :)