the google photos option is for the "instant upload" feature for the Google + app to be able to sync, while the other is for your regular web albums via Picasa...
per google:
Android 4.0 devices have a system setting that allows you to Sync Google Photos.
This setting enables syncing of your Google Photos from your web albums to your device's Gallery app. As a result, if you have a large number of photos in your web albums that have been uploaded through Picasa Web Albums or Google+, you may notice a high amount of battery or data usage during this sync.
This setting also enables or disables sync for Instant Uploads to Google+. However, to enable Instant Upload, you must enable Sync Google Photos as well as enable Instant Upload within the Google+ app.