Magnesium etches faster. That is not only a throughput advantage; it is a quality one, because the less time the etchant spends in contact with the plate, the less time it has to work sideways under the resist. Fine detail survives on magnesium partly because the process is shorter.

Zinc is coarser and cheaper, which suits work where the detail is moderate and the plate is a consumable consumed in quantity. Banners, calendars, general letterpress and board embossing are all zinc jobs, and choosing magnesium for them is paying for detail that the finished product will not show.

Weight is the difference nobody mentions until a large-format die has to be lifted into a press repeatedly. Magnesium is substantially lighter, and over a shift that is felt by the person doing the lifting.

The practical test is the smallest feature in the artwork. If the finest rule or the smallest point size would be visibly rounded by a slower, coarser etch, the job is magnesium. If it would not, the job is zinc and the saving is real.