Should I use the Gold or Silver Nisab?
The nisab calculated with the silver standard is significantly lower than its gold counterpart. This is because the value of silver has plummeted since the time of the Prophet (Allah bless him and give him peace).
There are arguments for using either nisab value - many scholars say that it is better to use the silver nisab since it will increase the amount of charity distributed, others say that the gold nisab is closer to the nisab in use at the time of the blessed Prophet (Allah bless him and give him peace).
However, if your assets consist entirely of gold, you must use the gold nisab, and similarly, if they consist entirely of silver then the silver nisab must be used.