I need to move a 7TB qtree from one filer to another (both filers are in a 7-mode cluster.) I have always done this at the volume level with snapmirror, I have never done it with a qtree snapmirror. I am assuming an async qtree snapmirror is faster then an ndmp copy (plus it stays in sync), and that those are in turn much much faster than rsync or tar or something done at the OS level.
The filers are connected with 1Gb ethernet, so the copy is going to take a very long time, the advantages of letting snapmirror "supervise" the whole thing are overwhelming, AFAIK.
Yes, QSM is a big win over rsync, particularly with one respect: snapmirror seems to keep a list of "dirty" inodes so it doesn't have to spend what can be hours looking for changed files like rsync does. This makes it orders of magnitude faster than rsync for incremental updates.
At least on a 2Gb etherchannel, the initial sync-up is no faster than other methods, if you can get them to work at wire speeds in both directions. Which almost never happens..... so QSM for the win again.