Honestly, backup and sync are really two different things. For a backup app you don't want it to sync deletes because it won't protect you against accidental deletes. An app that syncs (deletes and all) is good for keeping two (or more) devices up to date.
The iCloud Photo Library beta is probably a good choice for syncing... it will sync deletes and if you have multiple iOS devices it will keep them all up to date. You can select the option to keep an original copy of each photo on your device as well as in the cloud, so even with one device you'll still have two copies of each photo.