Rule of thumb on the step drill bit is it should be used on metal half as thickness of each step. In the pics listed, the long thin would be used for thicker metals because each step is thicker. The fat wide bits are for thinner sheet metal. Keep in mind, the cutting edge is the thickness of the step.
It will go through metal that is thicker, but if you are looking for a clean round hole, the above will ensure it.
Knockout punches are expensive when you get into larger sizes and they require you have a large hole for the main bolt. For a 3/4" hole you have to drill a 1/2" hole, so you are back at either trying to 'step' drill using standard twist drill bits, or using the step drill bit.