The nisab was set by Prophet Muhammad (peace be upon him) at a rate equivalent to 87.48 grams of gold or 612.36 grams of silver. It acts as a threshold by which to compare your net worth.
Choose gold as nisab if you have assets equivalent to or more than its recommended value. Otherwise, choose silver. If you have a mixture of both and can't decide, then it is recommended to go with silver. Read more