dc42:
Fair enough, however I didn't say anything about menus. Why not use a 128x64 GLCD for the display, then you can display the set points and actual temperatures all the time? If you use potentiometers and don't display the set points, then you'll have to calibrate the dials on the potentiometers.Rotary encoders are elder-friendly, you can make 1 click = half a degree, or whatever other increment you want.
This would be a very good approach for a user interface. Rotorary encoders will be a bit more expensive per part than push buttons or potentiometers; but they can be used in a much more intuitive way and can directly output a digital signal based on how much they are turned, and in which direction, to the Arduino. In contrast potentiometers will each need an analog input and, as dc42 mentioned, have to have the voltage range calibrated to the possible range of setpoint temperatures.