Hi @MOHIT_SHARMA4,
There is a different in terms of when they are executed. There is a good thread from 2021 here that gives a good explanation of the difference. The documentation also gives some benefits of runtime fields here as well.
To determine which one you use I would recommend having a look at the compromises in the documentation for their impact on searching to get an idea of some useful best practices and tradeoffs.
Hope that helps!