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.

No, because 120V RMS is the AC voltage that produces the same heat as 120 VDC. Theoretically it should be no difference at all.

For heating, the only thing that matters is active power. If the load is a resistor, active power is \$ V^2/R \$ with V being the RMS voltage.

The RMS value of 120V DC is 120V RMS, which will give the same active power in a resistor than 120V AC RMS.

AC power will be pulsed, but a 750W resistor will have enough thermal mass to smooth it out, so there is no difference.