Lowest price EVER for the STC-1000 on Amazon: $17.15 !!
Plus, how to reprogram it with upgraded software: http://hackaday.com/2014/03/15/temperature-controller-gets-open-source-firmware-upgrade/
DS18B20+ is easiest for Arduino because the analogue to digital conversion is already done for you. Also the 1 wire network is simple to set up and use and works over relatively long distances with no problems.
If your pump is brushless, like most are, PWM is a bit hit and miss. In my experience with a little tan pump PWM around 1 kHz does work but makes a horrible noise. At higher frequencies (like 20 kHz) it doesn't work.