Directory Synchronization vs. Migration

Though it can be used to bulk load objects into a target domain, the directory synchronization is (practically speaking) primarily for keeping users' passwords and group memberships up to date between source and target before you cut your users over.  It can also help to make sure that any net new objects created in the source during the migration, get created in the target.

If you use it for bulk loading objects into a target domain you cannot:

Be selective about where (what Target OUs) the objects are created in.  With a migration session, each batch of objects can be created in a different target OU.

You cannot perform "transformations" on the objects when they are re-created in the target - for example, applying a new naming convention for samaccountnames.

Hope this helps.

In short, a migration session can only add objects and attributes while you wait. A Sync session can create objects, deleted objects, write attributes and delete attributes. 

The "while you wait" part that Jeff mentions is an important detail.

Migration sessions are launched by a user.  Directory synchronization runs on its own schedule on a continuous start/stop basis.

The use of one approach or the other is contingent on a number of factors that contribute to your process design.  They are not mutually exclusive either.

Another tidbit is that migration sessions can be undone while directory synchronization can not.   

Another tidbit is that migration sessions can be undone while directory synchronization can not.   

Good point!   This is why I sometimes refer to the directory synchronization approach as an "object pump".