When deciding between business rules and client-side scripts in Model-driven Power Apps, the best practice is to use business rules are much as possible and only resort to writing code when necessary. The following table lists some of the most important reasons you might choose one over the other.

Business Rules	Client-side scripts
Run business logic on load	Run business logic on save
Set field values in form	Instantly update fields in form
Show / hide fields in form	Fetch related data