Delete
As you said, instance will lose the content permanently but may not be able to recover older content because of version purge [0]
Deactivate
You keep the content only on author which you may later need to use/modify/publish. As far as accidentally reactivating is concerned, you can easily tackle with permission and hooking a workflow where users will not be able to directly activate any page. All page activation can be directed via a governed approval process where right owner can verify.
Use deactivate + permission + workflow
[0] https://helpx.adobe.com/experience-manager/6-3/sites/deploying/using/version-purging.html