Is DC heating faster than AC heating?
When you say 120V @ 50Hz AC you are implicitly saying 120Vrms.

The RMS voltage is qualitatively defined as the voltage which will give the same resistive heating (averaged out over time) as a DC voltage of the same number. Therefore, by the definition of RMS, the heating will be the same because the RMS voltages are the same.

If you said 120Vpeak or something different, then things would be different.

This is in reference to a heater modeled only as a resistor. No extra real-world components like motors for fans.