Replication uses ZFS snapshots. Rsync uses the rsync protocol. Both can transfer data to remote systems using "deltas", ie. differences (additions, deletions, modifications etc.) so that you minimise the amount of data transferred across the network.
If you are transferring data to another ZFS system, then Replication would make the most sense, but if you are transferring data to a non-ZFS system then rsync is your only option.
Howdy Cyberjack.. I wager to guess you're the proprietor of this fine digital establishment.. Perhaps you might have a means of auto-closing discussion on old topics? In the case of this one, the question would seem to remain a current question had by many... Annnnyway.. Moving on...
I'm not the proprietor, just one of the highest users of the forum. I don't like to auto-close old discussions because sometimes people show up with alot of very insightful knowledge of a situation that has plagued users for months. Leaving threads open lets people add information. I trust that users can refrain from necro'ing dozens of threads on accident ;)
I've necro'd a thread or two on accident. Almost did it the other day. I just try to make sure we don't have a forum of new posts asking or answering questions for very old FreeNAS versions that have had those issues/bugs resolved already.