Silver plating is the process of putting a thin silver layer over an object. Silver plating is usually done over base metal, but it is possible to silver-plate nonmetallic items as well.
Nickel plating is the process of putting a thin nickel layer over an object. Nickel plating is also usually done over base metal, but it is possible to nickel-plate nonmetallic items.
Many people are allergic to nickel and avoid nickel-plated items. For persons with nickel allergies, silver plated items are preferred.
The cost of plating depends on the cost of the metals used. Although the metals markets fluctuate daily, silver usually costs more than nickel. It is, therefore, usually more expensive to plate an item with silver than with nickel.
Silver plate looks like sterling silver. Nickel plate, although it has a silver tone, looks less like sterling silver. People who want the look of sterling without the cost often select silver plate.