iCloud: iCloud Photo Library or iTunes?
You can use iTunes to sync photos and videos between your iOS devices and Mac or use iCloud Photo Library to keep your library—including any edits you make—up to date automatically on all your devices that are set up with iCloud.
Important: Once you start using iCloud Photo Library, you can’t use iTunes to add photos and videos to your iOS devices. Once you turn on iCloud Photo Library, any photos and videos synced using iTunes are removed from your iOS devices. If you wish to store the original versions in iCloud Photo Library, upload them with your Mac using the Photos app (with iCloud Photo Library turned on) or by using the Photos app on iCloud.com.
iCloud Photo Library
Uploads your photos and videos to iCloud automatically whenever you are connected to the Internet via Wi-Fi. Your iOS device and your Mac don’t need to be connected to each other, as they do when you use iTunes.
Optimizes photos and videos so they take up less space on your devices, and keeps originals in their native formats, at full-resolution, in iCloud.
Updates edits, additions, and deletions to iCloud and your devices as you make them. Edits are nondestructive and can be changed at any time.
Updates changes you make to the organization of your library to iCloud and your devices.
iTunes
Syncs all your photos, or selected albums, from your Photos, iPhoto, or Aperture library. Or, you can sync a chosen folder on your computer.
Backs up your device’s data—including your photos and videos—to iCloud or to a local computer or storage device.
Note: Syncing and backing up are two separate processes in iTunes; syncing alone does not mean you’ve safely backed up your photos and videos.