Log backup vs differential backups
We're trying to define a backup policy. 


You are starting from the wrong end of things. 

Backups are only the tools that you use to support a Recovery Strategy. 

Start by working out - in conjunction with your Business partners; you cannot do this alone - how long you can take to get back a "broken" database (Recovery Time Objective) and how much data they're prepared to lose while you go about doing so (Recovery Point Objective). 

Once you understand these two constraints, you'll have a better feeling of what you need your backup tools to do.