The Variac is a better choice in my opinion. The Variac will preserve the sinusoidal waveform of the voltage input to the wall wart. The dimmer chops up the input waveform into pulsed pieces of the sine wave. The wall wart may possibly overheat if operated on the chopped up waveform. However, I understand that wall warts with UL listings are designed and tested not to be a fire hazard. I'm not saying it won't work, I'm just saying the Variac is a more appealing solution.