How should I decide between throwing a new exception versus re-throwing an existing one in C#?

You should throw a new exception when you want to wrap an inner exception with more specific context or to change the exception type to something more appropriate for the current layer of your application. This is often done using the `throw new CustomException('Error message', innerException);` pattern. Re-throwing the original exception (`throw;`) is generally preferred when you want to preserve the original stack trace and exception type, typically when you've performed some logging or cleanup within the catch block and want the exception to continue propagating up the call stack.