What is the difference of backing up photos using Google Drive vs Google Photos?
That is a great question. You can automatically back up photos on your smartphone to the cloud using either the Google Drive app or Google Photos app. In either case you must have the back up feature turned on.
I typically recommend Google Photos for photo back up instead of Google Drive because it provides free unlimited photo storage when you set the back up at “high quality”.
With your Google Drive account you get 15 GB of free storage. If you use the Google Drive app for photo backup, the storage used counts against your available Drive space.