I think that measuring the temperature by the delta of the heating element is going to be rather difficult, and will require much more circuitry than a simple thermistor or other sensor would take.
1) You will need a calibration step to establish the baseline resistance at some reference temperature. (so you need a sensor anyway)
2) If you're trying to measure the resistance by current/voltage, you will probably find that the significant information is several orders of magnitude smaller than the measured values.
3) When the element is on, the measurement will only show the temperature of the element itself. If you need to know the present ambient, you will need to turn off the heater long enough for it to cool down to ambient. And you won't know when that is unless you have a separate temperature sensor.