NAS to NAS uses rsync (port 873 by default) and RTRR uses the rtrr (port 8899 by default) or the ftp protocol (port 20/21 by default). RTRR over ftp have some functional limitations compared to rtrr.
NAS to NAS should really be read QNAP to QNAP as that is the intention of it. The Rsync tab should be used with any non-Qnap rsync server as destination.
NAS to NAS works block based while RTRR works on full file level only.
In my experience rsync is almost always preferable; it is available on all major operating systems, it is very lean over slow connections, it doesn’t necessarily need to synchronize complete files when something have changed.