SnapMirror will preserve snapshots if that is the goal so "could" be better if you want a history of the data... but usually not an issue and I most often use ndmpcopy. With ndmpcopy, clean up (delete) the restore_symboltable file after the cutover is done, and for snapmirror, delete the snapmirror snapshots when done to clean up the migration.
So...if you don't need snapshot history, just ndmpcopy. For snapshot retention you can either snapmirror or vol copy (vol copy can copy all snaps the same as a baseline snapmirror initi or just a single snap)
Without snapshots (just the one ndmpcopy creats) ndmp might be a little faster but the copy is quick... using about 40MB/sec on the loopback adapter a 13GB (full volume) would be about 5 1/2 minutes... so either method mirror, vol copy, or ndmpcopy will not take much time.