I believe rebuild DELETEs the catalog first too, then create it from scratch
just like REBUILD INDEX
Optimize (like Reorganize Index) doesn't
Books Online (hit Help on the full-text screen)
http://msdn.microsoft.com/en-us/library/ms189385.aspx
Catalog Action
None
Does not perform Optimize catalog, Rebuild catalog, or Repopulate catalog operations.
Optimize catalog
Optimizes the space utilization of the catalog and improves query performance. It also improves the accuracy of relevance ranking of search results.
This action executes ALTER FULLTEXT CATALOG catalog_name REORGANIZE.
Rebuild catalog
Deletes and rebuilds the full-text catalog. This operation must be performed if a fundamental catalog property is changed, such as accent sensitivity.
For the rebuild to succeed, the filegroup the full-text catalog resides in must be online or read-writeable. After the rebuild, the full-text index will be repopulated.
This action executes ALTER FULLTEXT CATALOG catalog_name REBUILD.
Repopulate catalog
Updates the catalog with recent changes to the data. This option does require catalog downtime.