Google Photos in sync is what synchronizes your photos saved in Google Photos with the device. So if you add a photo on one device, it automatically appears on your other devices. This is only going to use data when photos are added/removed from Google Photos, so it shouldn't really be a concern for the battery unless you're constantly uploading photos to Google Photos from other devices or the computer.
Google Photos Backup is the automatic photo backup feature that uploads your pictures/videos to Google photos automatically. You can disable this, but you have to go into the Google Settings app. In there go to Google Photo Backup and you can turn it off or set it to upload only on wifi or mobile as well, and you can even set it to run the backup only when charging. And this will only be contributing to battery usage while photos are being uploaded, so unless you are taking tons of photos and have them set to upload over mobile or wifi.
Now the decision is yours!