I would the built in back up software to be a little more robust. When backing up from a source , it will only copy what is new or modified - which is good. But also an option to automatically remove files from the destination if they no longer appear in the source location.
ie. if you use your NAS like me for back up purposes from a media only hard drive , I sometimes clean up the source drive and move stuff around. The NAS box copies the new data that I've moved but doesn't delete the old data. - Does that make sense.