The differences between all three of those models are the attachments that are included with them and the switch. The performance of all three are the same 180/250 mph.
BV5600 includes the vacuum tubes, blower tubes, concentrator, reusable collection bag, leaf blaster attachment, and two speed switch.
BV6000 includes the vacuum tubes, blower tubes, concentrator, diffuser, disposable bag, detachable strap, and two speed switch.
BV6600 includes all the same attachments as the BV6000 plus a rake attachment and the switch is variable speed.