Which table to apply the business rule on if you have to populate data into one from another

Hi All,

New to servicenow and business rules. 

We have two tables - primary and secondary. Based on the drop down value selected in primary field, I have to create a new or update existing record in the secondary table to show that field in the secondary table.

1. Should I create the rule on the primary table in this case? 
2. what would be the best way to get the values into the related fields in the secondary tables.. 

Thanks!