5.9Lookup vs Master-Detail

Don't go too deep unless asked.

Lookup

Looser relationship.

Conceptually:

child ─────→ parent

The child may often exist more independently.

Master-Detail

Stronger parent-child ownership relationship.

The parent can influence things such as:

ownership
sharing
roll-up summaries
lifecycle behavior

depending on configuration.

For Mule integration work, what usually matters first is:

What field links these objects?
What Salesforce Id must I supply?