The single transistor driver will "lock" on to a frequency that changes depending on the length of the arc. This driver gets bring very quickly. The 555 based driver allows you to choose the frequency and duty cycle you are driving the MOSFET with. Depending on what frequency you tune the 555 based driver it can produce some pretty decent arcs at even just 12v input; The ZVS is a highly efficient zero volt switching based oscillator. The voltage from the flyback can vary with this driver but it gives high current on the output meaning the arc can be stretched out. At 12v input you will get under 10kV at around 8mA. But with 36v input mine will start arcing at 1 inch and can be stretched out to 5 inches meaning there is lots of current flowing through the arc.