[Entire file or parts of file?]

Most of my files are video editing projects (500MB-10GB file size) and photos (20-50MB file size), so having differential sync would save a huge amount of time and bandwidth as it wouldn't have to send a whole 2GB file after I changes a small part of it

[Sync between devices]

There are already plenty of great free programs out there to sync between local drives, I don't know if I'm allowed to name them on here, but you can just search google for 'file sync software'

Bittorrent sync is as the name says, a piece of software to keep multiple devices in sync with eachother

Syncing is not a backup, because if you delete a file from your computer, then it dissapears from the backup as well and you've lost that file. With backup software when you delete the file from your computer, it retains a copy of the file on the backup drive for X number of days, or forever if needed