I say thermostat.
If your room temp stays constant a rheostat might work for you. If the ambient temp changes you may need to change the setting. Another thing that can affect a rheostat is the available power, like if there is something else plugged into the same line or it's on a cheap extension cord.
A thermostat solves the problems, it monitors the temperature and and regulates the heat automatically to maintain the target temp.
I've never used flex-watt, I don't think it has different requirements than any other heaters and it should be compatible with a rheostat. If I'm wrong we'll soon know.