I read about both methods in Unity and wanted to ask about which was the best in efficiency : destroying the object or deactivating it. What is the performance difference?
Destroying the object gets rid of it completely. You cannot get it back. It is gone.
Deactivating it just disables it; everything is still there, it just does nothing.
Therefore, if you want to reuse the object, you can deactivate it, but if you will never use it again, you should destroy it (and get back its memory).
You should also mention that creating objects isn't cheap performance-wise. So you should be reusing objects that you plan to have multiple of. In general any game object that is being created and destroyed over the course of the whole game should be reused and not destroyed.
You can google object pooling for more information.