Scenarios where Service Policy should be used over iRule and Vice versa

This largely comes down to what you're used to.
Some people prefer using Policies for straightforward logic, turning to iRules only when more complex behavior is required.
Others, however, will use iRules regardless of the situation.

Policies are generally easier to configure and maintain, especially for those with less experience—which can be a key factor in long-term maintainability.
On the other hand, iRules offer nearly limitless flexibility, making them ideal for advanced use cases.